What is Language?
Before we can think about language acquisition, we have to think about what language means. What is a language and what is not? Is English a language? What about the sign language deaf and mute people use? Or the grunts Neanderthals presumably used instead of words?
Ok, here is my answer: Language is any means of communication on which a certain range of people can agree. That means English is a language and the signs and grunts are languages as well. A language does not need letters and written words. It does not even need sounds.
This may seem weird at first but think about it for a while. Language is nothing else than a means of communication. There is no rule that people have to communicate with sounds, even with comprehensible words. They can communicate with signs as well, be it body signs or drawings.
It is just that in our modern world, most languages are based on sounds forming words and sentences and have a set of letters or signs going with the sounds. Thus, we have a written language and a spoken language.
What Is Language Acquisition?
When we assume that language is nothing more than a means of communicating, then it can be said that language acquisition is nothing more or less than learning how to communicate.
Babies for example learn their first language because they see a need to communicate with their parents. “Wild children” who grew up with animals do not know how to speak like other humans but they can communicate with their “parents,” the animals that raised them.
Is it important to be perfect to communicate? No. Are you able to communicate only by knowing all the grammar rules of a language? No. Will you be understood if you only know some words without any grammar? Maybe to a certain extent. So it is important to get a good mixture of vocabulary and grammar to communicate without narrow limitations. And language acquisition is exactly that. To learn enough words and grammar rules to be able to communicate with the people around you.
You see, there is no such thing as a set definition of language acquisition. Everyone sets his or her own definition. If you want to go to Spain on vacation and want to be able to book a hotel room, order food, and drink in a restaurant and speak to a doctor if necessary, then language acquisition for you means to learn enough words and sentences and some basic grammar rules to cope with those typical situations. Nothing more. If you want to move to Spain and work for an IT company, for example, language acquisition for you means a completely different thing. You have to learn enough Spanish to be able to communicate with your colleagues, your neighbors, and a lot of other people on a daily basis. Furthermore, you have to know the specific terms used in the IT sector.
Now, what is language acquisition? Language acquisition is learning as much of a language as you need to communicate. The how is unimportant. The communication is all that counts.