Real Dictionary

parenthesis
Parentheses are a pair of curved marks that you put around words or numbers to indicate that they are additional, separate, or less important. (This sentence is in parentheses.)
countable noun: A parenthesis is a remark that is made in the middle of a piece of speech or writing, and which gives a little more information about the subject being discussed.

parenthesis in American English
an additional word, clause, etc. placed as an explanation or comment within an already complete sentence: in writing or printing it is usually marked off by curved lines, dashes, or commas
noun: either or both of a pair of signs ( ) used in writing to mark off an interjected explanatory or qualifying remark, to indicate separate groupings of symbols in mathematics and symbolic logic, etc

parenthesis in British English
noun: a phrase, often explanatory or qualifying, inserted into a passage with which it is not grammatically connected, and marked off by brackets, dashes, etc