Buffer
Definition
A solution that resists pH changes when small amounts of acid or base are added: made from a weak acid and its conjugate base.
Technical definition
A buffer is a solution of a weak acid and its conjugate base (or a weak base and its conjugate acid) that resists pH change upon addition of small amounts of strong acid or base. Its pH is given by the Henderson-Hasselbalch equation, and it works best within 1 pH unit of the pKa.
Example
Blood is buffered at pH 7.4 by the carbonic acid/bicarbonate system. Without it, normal metabolism would swing blood pH to lethal levels.
Common confusion
Buffers do not keep pH perfectly fixed and do not work forever. Add enough strong acid to consume all the conjugate base and the buffer breaks, exceeding its capacity.