| Definition -
A type of pronoun that is used when the sentence's syntax requires a subject but that subject doesn't exist.
For example, in the sentence It is obvious that this definition is incomplete, the it is a pleonastic pronoun because it doesn't refer to any existing subject.
Oxford English Dictionary - The term's first citation is from 1778
"A pleonastic pronoun."
(R. Lowth Transl. Isaiah (ed. 12) Notes 390)