1. a subunit or constituent part of something, often with the connotation that it cannot be reduced further. 2. a member of a set, class, or group. For example, in the set of all PhD-conferring American universities, any one of those American universities that confer the PhD is an element of the set. See also set theory. 3. one of the 118 officially identified chemical substances, of which 92 occur naturally, that cannot be reduced to simpler fragments by chemical means alone.