Wiktionary
n. (context grammar English) A pronoun that does not specify the identity of its referent.
Wikipedia
An indefinite pronoun is a pronoun that refers to non-specific beings, objects, or places.
Indefinite pronouns can represent either count nouns or noncount nouns and include a number of sub-categories: universal (such as everyone, everything), assertive existential (such as somebody, something), elective existential (such as anyone, anything), and negative (such as nobody, nothing).
They are associated with indefinite determiners (sometimes called indefinite adjectives) of a similar (or identical) form (such as every, any, all, some). A pronoun can be thought of as replacing a noun phrase, while a determiner introduces a noun phrase and precedes any adjectives that modify the noun. Thus all is an indefinite determiner in "all good boys deserve favour" but a pronoun in "all are happy".
Usage examples of "indefinite pronoun".
Corran realized, in one painfully crystal-clear moment, that in having used the indefinite pronoun it in his last command he had made a mistake equal in magnitude to still having all shield energy in his forward arc.