(physics and chemistry) the simplest structural unit of an element or compound
"A water molecule consists of two hydrogen atoms bonded to a single oxygen atom, representing the smallest unit that retains the properties of water."
The smallest particle of a specific element or compound that retains the chemical properties of that element or compound; two or more atoms held together by chemical bonds.
"A water molecule consists of two hydrogen atoms chemically bonded to one oxygen atom, retaining the unique properties of liquid water."
In plain English: A molecule is two or more atoms stuck together that act as a single unit to make up everything around you.
"You can see every molecule in that glass of water under a powerful microscope."
Usage: Use molecule specifically when referring to substances formed by two or more chemically bonded atoms, such as water or oxygen gas. Do not use it for single isolated atoms like helium, which are simply called atoms rather than molecules.
The word entered English via the French molécule and originally referred to a tiny mass. It was formed in Late Latin as a diminutive of moles, meaning "mass."