woman vs. female
Consider alternate spellings
labor vs. labour
16th century vs. sixteenth century
Consider broader and narrower terms
Great Britain vs. England
Homonyms. Remember databases and catalogs match characters, not concepts.
For example, the word "China": is it a country or a set of dishes? The computer can't tell!
Sometimes there is no universally agreed up term or phrase for a concept.
For example, middle ages vs. medieval, Native American vs. Indian, motion picture vs. movie vs. film
Avoid searching for long phrases or sentences. Extract key concepts instead.