Dictionary.net

1. A collection of four things having something in common. 2. In chemistry, a quadrivalent element. 3. In heredity, a bivalent chromosome which divides into four in the maturation division.

The number of four; a collection of four things.

An element with a valence or combining power of four.

A group of four similar bodies.

An element having a valency of four; a quadrivalent element.

A group of four micrococci.