1. adjectiveIn games such as football and hockey, when an attacking player is onside, they have not broken the rules because at least two players from the opposing team are between them and the goal when the ball is passed to them.
2. adjective [verb-link ADJECTIVE]If a person or group of people is onside, they support you and agree with what you are doing.
The company is phoning shareholders in an attempt to bring them onside.