From SKYbrary Wiki
Slip is an aerodynamic condition of uncoordinated flight in which an aeroplane moves towards the inside of a turn or is moving somewhat sideways as well as forward relative to the oncoming airflow.
A slip can either be intentional or unintentional depending upon the circumstances. Unintentional slip will usually occur during an uncoordinated turn in which the pilot has failed to make a sufficient rudder input. Intentional slip, either forward slip or sideslip, is an intentional cross control manoeuver in which the pilot has made an aileron input in one direction with a simultaneous rudder input in the opposite direction. This manoeuver is used to increase the rate of descent without increasing airspeed or to maintain the runway centreline during a crosswind landing.