A slip occurs when the bank angle of an airplane is too steep for the existing rate of turn. Unintentional slips are most often the result of uncoordinated rudder/aileron application. Intentional slips, however, are used to dissipate altitude without increasing airspeed, and/or to adjust airplane ground track during a crosswind. Intentional slips are especially useful in forced landings, and in situations where obstacles must be cleared during approaches to confined areas.
AFH Ch-08 (11) Intentional Slips
Cessna SOPM 2-100 Forward Slip to a Landing
Private Pilot ACS Page 32 Task M. Forward Slip to a Landing