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Wiktionary
only if

conj. not unless; ''used to introduce a necessary condition''.

WordNet
only if

adv. never except when; "call me only if your cold gets worse" [syn: only, only when]

Usage examples of "only if".

Separated from the bush, it looked unnatural, but only if one happened to have watched the whole process.

It comes into effect only if I fail to satisfy you completely over the course of our working together.