Acid
Definition
A substance that donates hydrogen ions (protons): sour taste, pH below 7, turns blue litmus red.
Technical definition
An acid is a proton (H+) donor in the Bronsted-Lowry model, or an electron-pair acceptor in the Lewis model. In water it increases the hydronium concentration, giving pH below 7. Strong acids dissociate completely while weak acids establish an equilibrium described by Ka.
Example
Hydrochloric acid in your stomach, at pH around 2, donates protons to help digest food.
Common confusion
Acids are not defined by containing hydrogen alone. Methane (CH4) has hydrogen but is not acidic: acidity is about willingness to DONATE the proton.