Used like 'sae' to emphasize an extreme example: 'even ...' (e.g., 'even a child'). It highlights that something applies in an unexpected or extreme case.
子供でさえこの問題を解けた。
Even a child could solve this problem.
彼は忙しくて、昼ごはんを食べる時間でさえない。
He's so busy that he doesn't even have time to eat lunch.
父でさえ驚くほどの変化だった。
It was a change that even my father was surprised by.
Attached after nouns or na-adjectives in conditional clauses to mean 'as long as ...' or 'if only ...' (e.g., 'as long as you are X, it's fine').
健康でさえあれば、他のことは心配しない。
As long as you're healthy, I won't worry about anything else.
君でさえ賛成してくれれば、この計画は進められる。
If even you agree, we can proceed with this plan.
静かでさえあれば勉強できるのに。
If only it were quiet, I could study.