The 9 Essential Terms to Pack on Your Journey into the World of Second Language Acquisition


Ciao! こんにちは (Konnichiwa)! Welcome to Adventures in Language. In this article, we’re going to make a packing list of sorts — we’re covering the 9 terms and concepts that you need to know to start your journey into the world of Second Language Acquisition. Now, you might find that some of these terms are familiar to you — but the way linguists use them might be new!

Here are the 9 Essential Terms to Pack on Your Journey into the World of Second Language Acquisition.

1. Second Language Acquisition

Second Language Acquisition is a scientific field that focuses on the learning and teaching of second and subsequent languages. It’s a subfield of linguistics, which is the scientific study of language. “Second Language Acquisition” is kind of a mouthful, so people usually just call it SLA for short.

Now, here’s something to look out for. Second language acquisition can also refer to the process of learning a new language. This meaning is synonymous with second language learning.

You can usually tell whether someone is talking about the field or the learning process from context. For example:

  • “Second Language Acquisition research can inform practices in language classrooms.”
    Here, we’re talking about the field.
  • “Second language acquisition takes more effort than first language acquisition.” 
    Here, we’re talking about the process. Got it?

You’ll also see a distinction between the two meanings in writing. Second Language Acquisition — the field — is capitalized, and is usually abbreviated as SLA. Second language acquisition — the process — is lowercase, and is usually abbreviated as L2 acquisition (where the L stands for . Language and the 2 stands for Second). So keep an eye out for that when you’re reading about language learning!

Now we’ve defined Second Language Acquisition — in two ways, no less! — but you might be wondering… what’s a second language?

2. First Language (L1)

The 9 Essential Terms to Pack on Your Journey into the World of Second Language Acquisition

Before we get to second languages, let’s start with first languages! A first language (L1) is any language learned very early in life. This is usually from birth — or even before that, in the womb! — but certainly within the first few years of life, up to around age 4. You might also hear the terms native language or mother tongue to refer to a first language. While a first language does not necessarily have to be spoken by a child’s mother, it’s one that a child hears from a parent, close relative, or caregiver — really, someone that they often hear speaking from a very young age. Many people around the world have multiple first languages. We tend to think of these people as bilinguals… but that’s actually a complicated term that we’ll come back to!

3. Second Language (L2)

In SLA research, a second language (L2) is any additional language that is learned after the first language(s). This might seem a little strange, because this means that a second language could be the second language an individual is learning… but it could also be their third, fourth, fifth, or even fifteenth language! Second languages might also be referred to as non-native languages, foreign languages, or target languages, or maybe even world languages in a classroom setting.

If a person has two first languages, the next language they learn will also be treated as a second language by SLA researchers… even though it’s technically their third! Things get even trickier when you think about young children who learn one language from birth and another from, say, age 2 — are these both first languages? Is one an early second language?

Are you confused yet? Don’t worry! It would probably be impossible to categorize all of the nuanced differences in the timing of language learning for different people. To keep it simple: a first language is a language learned in infancy and very early childhood, and a second language is any language learned after a first language… but just be aware that you might encounter some grey areas!

It would probably be impossible to categorize all of the nuanced differences in the timing of language learning for different people.

4. Acquisition

It’s time to talk about that A in SLA — acquisition. Acquisition is the process of developing a skill, which, in the case of SLA, is second language ability. To put it simply, acquisition is learning. However, in the early days of SLA, many researchers considered “acquisition” and “learning” to be two different processes (e.g., Krashen, 1982). Early attempts to distinguish between these terms have largely been disregarded, but you may still encounter literature attempting to uphold the distinction. It’s important to know that a historical distinction exists, but today these terms can be treated synonymously.

5. Learners

People who are learning (or acquiring!) second languages are referred to as — can you guess? Second language learners! The truth is that second language learning is, in many cases, a lifelong process, and even people who have been learning a second language for many years may still consider themselves learners. You may also hear these individuals referred to as second language users, speakers, writers, or signers — this terminology places less of an emphasis on acquiring a language, and more on actually doing something with it.

So, how do second language learners learn? Terms number 6 and 7 deal with two crucial ingredients in the language learning process: input and output.

6. Input

Input is the target language that a learner is exposed to, either through listening, reading, or viewing (for signed languages).

7. Output

Output is the target language that a learner produces, through speaking, writing, or signing.

Input and output are two crucial ingredients for second language development.

The idea that a learner would need some exposure to target language input is probably fairly intuitive, right? How can you learn a language if you don’t have any examples to draw on? And so learners need rich, plentiful second language input, ideally from a variety of speakers and contexts.

However, research shows that input isn’t really enough when it comes to second language development (Swain, 1985, 2005). Learners really need to be pushed to produce language — that is, to produce output — which serves as important practice with the language and helps learners develop beyond understanding meaning to grasping complex grammatical concepts.

8. Proficiency

All of that input and output practice helps learners to improve their language proficiency! Proficiency is an individual’s level of ability in a language. It’s usually measured on a scale, with values on the low end like “low” or “beginner,” and values on the high end like “high,” “advanced,” or the coveted “native-like.” Many second language learners find themselves stuck in the very large middle ground of “intermediate proficiency,” often referred to as “the intermediate plateau.” Proficiency levels can also be different for different language skills. For example, you might have advanced reading proficiency in a language, but intermediate speaking proficiency. You are very likely to come across talk of proficiency when learning about SLA. Just know that it’s actually notoriously difficult to define and measure! 


Did you know? You can have different levels of proficiency in different language skills. That means you might have advanced reading proficiency, but intermediate speaking proficiency.

9. Bilingual

Our 9th and final term is bilingual. Most people would agree that someone who learns two languages from birth and speaks both of those languages fluently is bilingual. But what about if you started learning your second language as an adult? What if you’re still learning that second language? When can you call yourself bilingual? These are excellent questions, but they don’t really have decisive, scientific answers. In some ways, you have to decide how to best describe yourself.

Most linguistics researchers today would agree that bilingualism is a spectrum, including anyone who speaks more than one language. SLA and Bilingualism are two distinct fields of study with different origins and central ideas — SLA research focuses more on the process of learning additional languages and the challenges associated with that, whereas Bilingualism research focuses more on living with multiple languages. But they are highly interrelated — it’s not uncommon to see academic journals, books, courses, and even university departments with the name “SLA and Bilingualism.” Sometimes the same individual might be referred to as a second language learner or a late bilingual, depending on the focus of a research study.

When can you call yourself bilingual? The answer is complicated!

Well, there you have it! Your Top 9 essential terms to get you started on your Second Language Acquisition journey! We hope you feel prepared to embark on your next SLA adventure!


Thanks for reading!

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Fill out the form below  for some fun – and free – materials about SLA. We have some printable flashcards that you can use to study these terms on the go. We also made a version that’s compatible with online flashcard tools like Quizlet, if that’s your preferred study method! To embark on your next language learning adventure, visit us at!

Wondering what languages were used in today’s article? Ciao (CHAo) means “hello” and “goodbye” in Italian, a Romance language spoken primarily in Italy.こんにちは (konnichiwa) and じゃあ、また (jaa, mata) mean “hello” and “see you later” in Japanese, an East Asian language isolate spoken primarily in Japan.

References & Further Reading

Meet The Author:
Author - Kaitlyn Tagarelli
Kaitlyn Tagarelli
Linguist at Mango Languages
Kaitlyn Tagarelli (Ph.D., Georgetown University) is a linguist and the Head of Research at Mango Languages. She holds a Ph.D. in Linguistics from Georgetown University, specializing in how the mind and brain learn languages. Aside from geeking out about all things neuroscience and linguistics, she loves hanging out with her family at their Connecticut home, trying to convince them to speak French with her.

To embark on your next language adventure, join the Mango fam!

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