Plain English Meaning
One side of the airplane weighs more than the other, so it tends to lean or roll toward the heavier side.
Full Definition
A weight distribution in which the left and right sides of the aircraft are not equally loaded, causing the aircraft to be heavier on one wing than the other. This shifts the lateral center of gravity off the aircraft's longitudinal centerline.
Context Anchor
Encountered in weight-and-balance discussions when passengers, baggage, cargo, or fuel are not distributed evenly between the left and right sides of the aircraft.
Derivation
Lateral comes from the Latin lateralis, meaning 'of the side.' Unbalance simply means 'not balanced.' Together: side-to-side imbalance — distinct from longitudinal (nose-to-tail) imbalance.