What is the phrase called when it in parentheses?

The phrase in parentheses is called a parenthetical phrase or parenthetic (the adjective form).

Here are some other terms you might encounter:

* Interjection: This is a broader term that includes any phrase or word that interrupts the flow of a sentence. Parentheticals are a type of interjection.

* Aside: A similar term to "parenthetical," often used in literature, particularly drama, to indicate a character's whispered or unspoken thoughts.

It's worth noting that parentheses aren't the only punctuation used for parentheticals. You can also use dashes (-) or commas (,) for this purpose.