A skid occurs when there is too much rudder input for the amount of bank in a turn, causing the aircraft to slide sideways through the air toward the outside of the turn. This creates excess drag and pushes the slip indicator ball to the outside of the turn coordinator. If an aircraft stalls while skidding, it is highly susceptible to entering a dangerous spin.
Heard on the job
Step on the ball, you're in a skid and bleeding off too much airspeed in the pattern.
Insider tip
Pilots are taught to 'step on the ball' (apply rudder on the side the inclinometer ball is resting) to correct a skid and return to coordinated flight.