A strong acid is an acid that dissociates according to the reaction 
where S represents a solvent molecule, such as a molecule of water or dimethyl sulfoxide (DMSO), to such an extent that the concentration of the undissociated species  is too low to be measured. For practical purposes a strong acid can be said to be completely dissociated. An example of a strong acid is hydrochloric acid.
:(in aqueous solution)
Any acid with a  value which is less than about -2 is classed as a strong acid. This results from the very high buffer capacity of solutions with a pH value of 1 or less and is known as the leveling effect.
The following are strong acids in aqueous  and dimethyl sulfoxide solution. The values of , cannot be measured experimentally. The values in the following table are average values from as many as 8 different theoretical calculations.
Also, in water
* Nitric acid   = −1.6 
* Sulfuric acid  (first dissociation only, ≈ −3)
The following can be used as protonators in organic chemistry
* Fluoroantimonic acid 
* Magic acid 
* Carborane superacid 
* Fluorosulfuric acid ( = −6.4)
Sulfonic acids, such as p-toluenesulfonic acid (tosylic acid) are a class of strong organic oxyacids. Some sulfonic acids can be  isolated as solids. Polystyrene functionalized into polystyrene sulfonate is an example of a substance that is a solid strong acid.