Plain English Meaning
The aircraft is flying slightly sideways or crooked relative to the air it is moving through, instead of pointing straight into the airflow. This makes the air push harder against the body of the aircraft, which slows it down and wastes fuel.
Full Definition
A flight condition in which the aircraft is not aligned with its direction of travel through the air, causing the airframe to meet the relative wind at an angle rather than head-on. This misalignment increases parasite drag because surfaces that are normally edge-on to the airflow are now exposed broadside, requiring more thrust to maintain speed and reducing overall efficiency.
Context Anchor
Seen in stability discussions, especially when an is disturbed from and its surfaces meet the airflow at an angle.