Chemistry. A substance that brings about a chemical or physical effect or causes a chemical reaction. In later use chiefly with preceding modifying…

Chemistry. A substance or other agent that by reacting with another substance breaks it down to its component parts. Obsolete.

Chemistry. A substance capable (esp. in a finely divided state) of igniting spontaneously on exposure to air. Now historical.

Any substance capable of igniting spontaneously on contact with air or water; = pyrophorus, n. 1. Now historical.

An imagined ‘imponderable’ substance supposed to be the cause of cold. Cf. caloric, n.

A substance which travels by a process of diffusion; the portion of a mixture that passes through a semipermeable membrane during the process of…

A substance which diffuses through another.

A substance used to produce very low temperatures; a cryogenic substance. Frequently attributive.

Chemistry. A substance that changes colour when heated. Cf. metachromatism, n. 1a, metachromism, n. 1. Obsolete. rare.

Any relatively inert material used as a base or bulk medium with which a smaller amount of an active substance (such as a catalyst, drug…

Originally: (in vacuum technology) a substance used to remove residual or contaminating gas from a sealed and partially evacuated chamber, by…

A substance, molecule, etc., with particular chemical or biological activity; an active ingredient. Cf. sense A. 10.

(a substance containing) an atom which can become part of the network of atoms (chiefly silicon and oxygen) in a glass (see sense A. 2d).

(a substance containing) a metal ion which can occupy an interstice in the network structure of a glass (see sense A. 2d).

a substance that is used to increase the sensitivity of an organism or tissue to ionizing radiation.

Physical Chemistry. A substance or mixture consisting of a single phase.