There are several definitions of an acid (Arrhenius, Brönsted etc.) but the Lewis definition is the widest. This states that an acid is an electron pair acceptor (and hence possesses an empty orbital).
Conjugate acid and base
For every acid there is a conjugate base and vice versa. A base is therefore an electron pair donor. The strengths of an acid and its conjugate base are related. If an acid is strong, its conjugate base is weak, and reciprocally.
General facts about acids
An acid will colour litmus paper red and have a pH lower than 7.
General scheme for the reaction between and acid (A) and water. The chemical equilibrium in the reaction involves the conjugate base (B) of the acid. © DR