Plain English Meaning
When you bank the airplane, part of the lift no longer pushes straight up — some of it now pushes sideways. That sideways pull is what curves the airplane around the turn instead of letting it keep flying straight.
Full Definition
In a turn, the horizontal component of lift that acts toward the center of the turn, pulling the aircraft sideways out of straight-line flight and into a curved path. It is produced when the aircraft is banked, because tilting the lift vector splits it into a vertical component (which still supports the aircraft against gravity) and a horizontal component (which acts to the side).
Context Anchor
Seen in discussions of turns, especially when explaining how lift is split into an upward part and an inward sideways part during a bank.
Why Pilots Care
Understanding sideward force explains why a coordinated turn feels balanced and why or occur when and are mismatched.