"Herstory" is a neologistic term for "women's history," a variant of the Marxist "people's history"; while a people's history professes to reinterpret history from the perspective of workers and/or common men, a herstory professes to reinterpret it from the perspective of women. Most feminists don't use it.
The term is an illustration of its coiners' belief that regular history is heavily slanted toward men's point of view, a "systemic bias" reflected in the term history, which they seem to have simply assumed was a portmanteau of "his story".
Unfortunately, it happens that the English word history is a loan word, derived directly from the Latin historia, which is itself a loan word from ancient Greek. On the other hand, the English word his is derived from a proto-Germanic root, and is not in the least etymologically connected to the first three letters of history.
- Robin Morgan, who coined the term.