Initiate
The hero or skill that opens a fight, usually the first CC or dive of the engage.
To initiate is to be the first hero committing to a fight — landing the opening stun, jumping in first, or drawing enemy CC before teammates follow up. It's typically a tank's job (Tigreal's ultimate, Atlas's pull), since they can absorb the response damage that would kill a squishier hero for the same play. A fight where the wrong hero initiates — a marksman walking in first instead of the tank — usually ends badly, since enemy CC and burst lands on the least durable target.