pleonastic pronoun
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)

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