A slip is an aerodynamic state where an aircraft is moving somewhat sideways as well as forward relative to the oncoming airflow (relative wind).
An aerodynamic slip has several applications including rate of descent control, runway alignment, and compensating for a crosswind when landing.
The terms “Forward Slip” and “Side Slip” are commonly used. I find these terms confusing since a forward slip looks like you are flying sideways and a side slip looks like you are moving forward.
Aerodynamically, a slip is a slip is a slip.
|Flight Training Manual for Gliders – Holtz|
|Lessons||4.15||Side Slip – Correcting for Alignment Errors|
|4.16||Side Slip – Compensating for a Crosswind|