Definition:Parenthesis/Linguistic Note

Linguistic Note on Parenthesis
The plural of parenthesis is parentheses.

The correct pronunciation of parenthesis is par-en-te-sis (or par-en-the-sis), while parentheses is pronounced par-en-te-sees (or par-en-the-sees).

It also needs to be pointed out that US English uses the term parentheses to mean the brackets $( \ldots )$ themselves (specifically the round ones), rather than their content. The word brackets is generally reserved for square $[ \ldots ]$ and curly $\{ \ldots \}$ versions (although the technical term for the latter is braces).

While this is common in natural language, such usage is discouraged in, as it is more useful to have a word which can be specifically used to unambiguously refer to the content. It is also worth pointing out that use of parentheses to mean the brackets is considered by much of the rest of the world as ignorant.