Acculturation may be defined as the process in which someone learns a foreign culture, that is, a culture other than their own. Culture here includes moral values, behaviors, and—the focus of this article—language.
Acculturation most commonly occurs with a member of a minority culture being exposed to a majority culture, such as the situation with an immigrant moving into a new country. For language, this means there are two things to consider: the degree to which person learns the language of the new country, and the degree to which the old language is retained.
However, acculturation may also occur in situations where one culture is not clearly dominating the other in a mutual flow of language shifts. For instance, consider the number of foreign phrases that exist within the English language, from tornado to bon appetit! In such cases, people within one culture have to a degree acculturated to another, not necessarily dominating culture.
Sometimes, a third language may even be created out of the blend of the two cultures. Consider the prevalence of Denglisch, for instance, a blend of English and German, or of varieties of Creole.
This is to be differentiated from enculturation, which is slightly different. Enculturation has come to mean the process by which you learn your “home” culture and language, whereas acculturation generally refers to the process by which one learns a second culture and language. However, older works will still use the terms interchangeably, so be careful!
This must also be differentiated from assimilation, which is what occurs when one abandons their native culture and wholly adopts another.