(linear algebra) An element on the main diagonal of a square matrix, that is, an element in row k and column k where k is an integer between 1 and the number of rows (or columns) in the matrix.

(chemistry, geology) any chemical element which occurs in nature uncombined with other elements; includes the atmospheric gases, several metals and semimetals, and the nonmetals sulfur and carbon

In a situation which is entirely suitable, familiar, or enjoyable.

An element that is used to make up an alloy.

(chemistry) Any synthetic element having an atomic number greater than that of uranium.

(category theory) The identity element of an object when thought of as a generalized element of that object.

(category theory) A morphism whose codomain is some specified object.

(idiomatic) The strategic advantage afforded to a party when its opposition may be caught by surprise.

(category theory) A morphism from the terminal object to a given object (to which it is said to belong).

(geology) An element which is not a trace element in a given context, and which is present in significant quantity.