A colloqualism is an informal expression that is more often used in casual conversation than in formal speech or writing. You could say that a colloqualism for the word colloqualism is an “americanism”, however, that is similar to saying that we don’t speak English, we speak American. In any case, collaqualisms are a good way to transform your classroom learned English into that of a native-speaker.
Classroom teachings are a very important step in the learning process of English. But to truly step into the next level of speaking like an American, you need to learn and understand our colloquialisms.
•”Over the last generation or so writing has become more informal than it ever was before. The area of highly formal writing has shrunk considerably; it is now confined to state papers, articles in learned publications, commencement addresses (and by no means all of those), legal documents, court decisions, and prefaces to dictionaries. Other writing has become quite hospitable to so-called colloquialisms; it has become more informal, more relaxed, more familiar, more casual.”
(Theodore Bernstein, The Careful Writer, Simon & Schuster, 1995)
Needless to say, americanisms are basically slang words. I could go on all day about all the slang words but that just might be an impossible task. So here is a decent American slang site with a good list and a fun way to learn.

