An acid is a chemical compound that typically reacts by donating protons (H+) to another compound, called a base.
Some compounds, like water, can act either as an acid or a base, and are called amphoteric compounds. Stronger acids also typically oxidize metals, forming salts and releasing hydrogen.
Some of the stronger acids include the hydrohalic acids - HCl, HBr, and HI - and the oxyacids, which tend to contain central atoms in high oxidation states surrounded by oxygen - including HNO3 and H2SO4.
The Brønsted-Lowry definition revolves around an acid's ability to donate protons in a chemical reaction. The more general definition offered by Lewis describes the reactivity of an acid in terms of its ability to accept a pair of electrons from a base. In this more general sense, aprotic compounds (those which do not donate protons), can still react with bases, and the terms "acid" and "base" can still be used for reactions in aprotic or non-aqueous environements.
Acidity is typically measured using the pH scale.
"Acid" is also a slang word referring to LSD.