(1):

(n.) The state of lying with the face down; - opposed to supineness.

(2):

(n.) The quality or state of being prone, or of bending downward; as, the proneness of beasts is opposed to the erectness of man.

(3):

(n.) Descent; declivity; as, the proneness of a hill.

(4):

(n.) Inclination of mind, heart, or temper; propension; disposition; as, proneness to self-gratification.