molecula

molécula.

(Del dim. del lat. moles, mole).

1. f. Unidad mínima de una sustancia que conserva sus propiedades químicas. Puede estar formada por átomos iguales o diferentes.

~ gramo.

1. f. Cantidad de una sustancia química cuyo peso es su peso molecular expresado en gramos.
Fuente: Diccionario de la Real Academia Española

2. (Physics) The smallest part of any substance which possesses the characteristic properties and qualities of
that substance, and which can exist alone in a free state.
[1913 Webster]

3. (Chem.) A group of atoms so united and combined by chemical affinity that they form a complete, integrated
whole, being the smallest portion of any particular
compound that can exist in a free state; as, a molecule of
water consists of two atoms of hydrogen and one of oxygen.
Cf. {Atom}.
[1913 Webster]

Source: The Collaborative International Dictionary of English