Conjugate Acid
Definition
What a base becomes after it accepts a proton: the acid partner in an acid-base pair.
Technical definition
A conjugate acid is the species formed when a base accepts a proton (H+). Every Bronsted-Lowry base has a conjugate acid differing by exactly one H+, and strong bases have weak conjugate acids and vice versa.
Example
Ammonia (NH3, a base) accepts a proton to become NH4+ (its conjugate acid). The pair NH4+/NH3 is a classic buffer system.
Common confusion
Conjugate pairs differ by exactly ONE proton. H2SO4/HSO4- is a conjugate pair, but H2SO4/SO4^2- is not, since that differs by two protons.