(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.