Parenthetical

[PussyCat]

Of the nature of a parenthesis; pertaining to, or
expressed in, or as in, a parenthesis; as, a parenthetical clause; a
parenthetic remark.

[Sonia]

Relating to or inserted as a parenthesis.

[Tariq Zaki]

Extra [details] [inserted] into a sentence (spoken or written). Usually the details are just [relevant] enough to be included, even though everything would be perfectly clear (though possibly less interesting) without them.