The phrase "on off" can be used to describe a switch or button that controls the flow of power or activity to a device. There are several synonyms that can be used instead of "on off" depending on the context. Some common alternatives include "power switch," "toggle switch," and "control switch." Similarly, for activities, synonyms for "on off" could include "start stop," "begin end," or "commence terminate." As with any word, choosing the right synonym depends on the intended meaning and the surrounding language used in the sentence.