1. The quality or state of being prone, or of bending downward; as, the proneness of beasts is opposed to the erectness of man.
2. The state of lying with the face down; -- opposed to supineness.
3. Descent; declivity; as, the proneness of a hill.
4. Inclination of mind, heart, or temper; propension; disposition; as, proneness to self-gratification.
n : being disposed to do something; "accident proneness"