An Acid is a substance containing hydrogen in its molecules (or ions) that it can release as H+ ions. The Bronsted-Lowry definition of an acid is that it is a 'proton donor'. By strength of an acid is meant the tendency with which it will donate H+ ions; the stronger the acid the more readily it will donate H+ ions. The terms 'strength' and 'concentration' with regard to acids must not be confused; a concentrated solution of a weak acid is still a weak acid.
Hydrochloric acid, HCl, is an example of a strong monobasic acid; it is essentially completely ionised in aqueous solution.
HCl(aq) + H2O(aq) ® H3O+(aq) + Cl-(aq)
The organic acid, ethanoic acid, CH3COOH, is an example of a weak monobasic acid; it is partially ionised in aqueous solution, an equilibrium existing between the acid and its conjugate base.
CH3COOH(aq) + H2O(l) = H3O+(aq) + CH3COO-(aq)
There is another common definition of an acid. It is the Lewis definition. A Lewis acid is an electron-pair acceptor.
See also Acid Dissociation Constant | Base.