Strong Acid
Definition
An acid that dissociates completely in water: every molecule donates its proton. The famous seven (HCl, HBr, HI, HNO3, H2SO4, HClO4, HClO3) are the ones to memorize.
Technical definition
A strong acid dissociates essentially 100 percent in aqueous solution (Ka far above 1), so [H+] equals the initial acid concentration for monoprotic acids. The seven common strong acids are HCl, HBr, HI, HNO3, HClO4, HClO3, and the first proton of H2SO4.
Example
0.10 M HCl gives [H+] = 0.10 M and pH = 1.00. No equilibrium calculation is needed because dissociation is complete.
Strong does not mean concentrated. A dilute 0.001 M HCl is still a strong acid (fully dissociated) while 12 M acetic acid is still weak (barely dissociated): strength is about dissociation, concentration is about amount.