double bond

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A covalent bond in which two electron pairs are shared between two atoms.

Wiktionary

1. n. chemistry A covalent bond in which two electron pairs (instead of the usual one) are shared between two atoms; most common between carbon atoms and carbon, oxygen or nitrogen atoms, but several other forms are known.

GNU Webster's 1913

1. n. a covalent bond which involves the merging of orbitals of two (or three) electrons on each of the two connected atoms, thus satisfying two (or three) units of valence on each of the bonded atoms. When two carbon atoms are thus bonded, the bond (and the compound) are said to be unsaturated.

WordNet 3.0

1. n. a covalent bond in which two pairs of electrons are shared between two atoms