The terms "strong" and "weak" are used to describe the extent to which an acid dissociates into ions in an aqueous solution.

• Strong acid: An acid that completely dissociates into ions in water. For example, hydrogen chloride (HCl) is a strong acid because it ionizes completely into hydrogen ions and chloride ions in water. 

• Weak acid: An acid that only slightly dissociates into ions in water. For example, acetic acid is a weak acid because it only dissociates about 5% into ions in water