Take, for example, the sentence "The dog, which is in the corner of the room, is barking.". Is there a specific term for the part of the sentence "which is in the corner of the room", which can be removed to leave the valid sentence of "The dog is barking."?
It is a Parenthesis. It derives its name from the punctuation marks called parentheses, curved brackets — one but not only device employed to mark off any dispensable amplifier be it a phrase, clause , word or sentence. They can be marked off without hampering the meaning of the main sentence or relevant sentence by such punctuation marks as round or square brackets, comas, or dashes.
Any modifier appearing after the noun or pronoun, an appositive, a noun in vocative use, asides in dramas or absolute phases or nominative absolute can safely be called a Parenthesis.
- The dog, which is in the corner of the room, is barking.
The portion marked of by comas can be so done by brackets or dashes as well. It is an example of parenthesis. The spelling is different from that punctuation mark, parentheses it lends its name to.