Cusp

The imaginary line which separates a Sign from adjoining Signs, a House from its adjoining Houses; (b) an indeterminate but small arc contiguous to the boundary-line between adjacent Signs and Houses, wherein there is 
uncertainty as to the planet’s location at a particular moment, and ambiguity as to the planet’s influence in a borderline 
relationship. A birth planet is stronger when it is on the cusp than when it is in the last degrees of a House. The angular 
cusps are doubtless the sharpest.

DeVore, Nicholas. Encyclopedia of Astrology. New York: Philosophical Library, 1947.