Real Dictionary

comma
countable noun: A comma is the punctuation mark , which is used to separate parts of a sentence or items in a list.

comma in American English
a mark of punctuation (,) used to indicate a slight separation of sentence elements, as in setting off nonrestrictive or parenthetical elements, items in a series, etc.
noun: the sign (,), a mark of punctuation used for indicating a division in a sentence, as in setting off a word, phrase, or clause, esp. when such a division is accompanied by a slight pause or is to be noted in order to give order to the sequential elements o

comma in British English
noun: the punctuation mark(,) indicating a slight pause in the spoken sentence and used where there is a listing of items or to separate a nonrestrictive clause or phrase from a main clause