What is a Diatomic Molecule? Is HCl a diatomic molecule?

Diatomic molecules are molecules composed of only two atoms, of the same or different chemical elements. 

A diatomic molecule consisting of two atoms of the same element, such as hydrogen (H2) or oxygen (O2), is said to be homonuclear.

A diatomic molecule consists of two different atoms, such as carbon monoxide (CO) or nitric oxide (NO), the molecule is said to be heteronuclear.

HCI is a Heteronuclear Diatomic Molecule.