Conjugate Base
Definition
What an acid becomes after it donates a proton: the base partner in an acid-base pair.
Technical definition
A conjugate base is the species remaining after an acid donates a proton (H+). Every Bronsted-Lowry acid has a conjugate base differing by exactly one H+, and the stronger the acid, the weaker its conjugate base.
Example
Acetic acid (CH3COOH) donates a proton to become acetate (CH3COO-): its conjugate base.
Common confusion
The conjugate base of a strong acid is an extremely weak base. Cl- from HCl is such a weak base it is effectively neutral in water, which is why NaCl solutions are not basic.