Weak Acid
Definition
An acid that only partly dissociates in water: most molecules stay intact, setting up an equilibrium described by Ka.
Technical definition
A weak acid dissociates incompletely in water (Ka below 1), establishing the equilibrium HA forming H+ + A-. The [H+] must be found through the Ka expression, and weak acids are the acid components of buffers.
Example
Acetic acid (Ka = 1.8 x 10^-5) in vinegar: a 0.10 M solution has pH about 2.87, not 1.00, because only about 1.3 percent dissociates.
Common confusion
Weak acids still follow the same pH math. You just cannot assume [H+] equals the starting concentration: that is the whole point of the Ka calculation.