Plain English Meaning
Trim is the act of adjusting small control settings so the airplane flies hands-off in the attitude you want, without you having to push or pull on the controls to hold it there.
Full Definition
The condition in which an airplane maintains a desired attitude and flight path without the pilot having to hold continuous pressure on the flight controls. Trim is achieved by adjusting trim devices (such as trim tabs on the elevator, rudder, and sometimes ailerons) so that aerodynamic forces hold the control surfaces in the the pilot wants, relieving .