Coefficient
Definition
The large number written in front of a formula in a balanced equation. It tells how many molecules or moles take part.
Technical definition
A stoichiometric coefficient is the integer placed before a chemical formula in a balanced equation, indicating the relative number of moles (or molecules) of that species involved. Coefficients set the mole ratios used in all stoichiometric calculations.
Example
In 2H2 + O2 -> 2H2O, the coefficient 2 before H2 means two moles of hydrogen react per mole of oxygen.
Common confusion
Coefficients are not subscripts. The 2 in 2H2O means two whole water molecules; the subscript 2 in H2O means two hydrogen atoms inside one water molecule. Never change subscripts to balance an equation.