That is either fully on or fully off, and is used to toggle a status between on and off; as in an on-off switch / That fluctuates between a positive and negative status, as an on-off relationship

(transitive, idiomatic, US) To supervise a group of people and their work.

(chemistry, physics) A chip, combining semiconductor and microfluidics technologies, capable of handling and analyzing very small quantities of sample, especially of bodily fluids for medical purposes.

(idiomatic) behaving violently or rioting

(Britain) A secondary, often unintended effect; a repercussion, chain reaction.

working, busy, at work / (slang) Having sex

(intransitive) To realize; come to understand.

A form of challenge, in the same spirit as a medieval knight throwing down his gauntlet.

A contest involving only one player on each side, especially of an activity often involving teams.

(chiefly Canada, US) Involving direct communication or competition between two people. / (sports) Involving one attacker and one defender. / (mathematics) bijective or injective