Molecule
Definition
Two or more atoms joined by chemical bonds, acting as a single particle. Water is made of H2O molecules.
Technical definition
A molecule is an electrically neutral group of two or more atoms held together by covalent chemical bonds. It is the smallest identifiable unit of a covalent compound that retains the compound's chemical properties.
Example
One molecule of water contains two hydrogen atoms and one oxygen atom bonded together. A single glass of water holds roughly 10^24 of them.
Common confusion
Not every substance exists as molecules. Table salt forms a crystal lattice of ions rather than discrete NaCl molecules, so chemists use the term formula unit there instead.