Philosophy. In the Scholastic philosophy: The essential determinant principle of a thing; that which makes anything (matter) a determinate species or…

A substance used or acted upon in a physical operation; a substance undergoing a physical or chemical process. Obsolete.

Forming the substratum or substance. Chiefly in matter subject: = subject matter, n. Obsolete.

Chiefly Philosophy. The inherent nature or essence of a person or thing; what makes a thing what it is. Cf. haecceity, n.