A meaningful strength or size difference between two sides of the body, or between a muscle and its opposite, large enough to change how a client moves.
Small differences are normal. Most people are 5 to 10 percent stronger on their dominant side and it never causes a problem. The threshold worth acting on is around 10 to 15 percent, or any difference the client can feel during a lift.
Find it with unilateral work, because a barbell hides it. If a client presses 24 kg for 10 with the right arm and 18 kg for 10 with the left, that is 25 percent and it is worth a block of attention. The fix is not extra sets on the weak side: it is doing the same number of sets on both sides and letting the weak side set the reps, so the strong side does 3 x 8 at 18 kg while the weak side catches up.
Front-to-back imbalances are handled the same way, by adding volume to the underworked pattern rather than removing it from the other.