A spline is a ridge or tooth[1][2][3] on a drive shaft that matches with a groove in a mating piece and transfers torque to it, maintaining the angular correspondence between them.
For instance, a gear mounted on a shaft might use a male spline on the shaft that matches the female spline on the gear. Adjacent images in the section below show a transmission input shaft with male splines and a clutch plate with mating female splines in the center hub, where the smooth tip of the axle would be supported in a pilot bearing in the flywheel (not pictured). An alternative to splines is a keyway and key, though splines provide a longer fatigue life, and can carry significantly greater torques for the size.[2]
The externally splined shaft mates with an internal spline that has slots, or spaces, formed in the reverse of the shaft's teeth.
1966 G. W. MICHALEC Precision Gearing vii. 324 Generally, involute internal and external teeth are mated, but non~involute splines are also suitable. 1979 Industrial Fasteners Handbk. I. 318 There are two basic forms of spline--straight-sided splines which may number 4, 6, 10 or up to 16 splines equally distributed around the circumference of a shaft, and serrated splines which are in the form of adjacent triangular teeth.