The suffix ity is often used to change an adjective into a noun.
real + ity = reality
For most adjectives ending in 'e' we drop the 'e' before the suffix is added.
pure + ity = purity
However, if the adjective ends in 'ble' then the 'le' needs to be dropped and ility is added.
possible + ility = possibility
capable + ility = capability
Take care, though, as there are always exceptions to rules!
The noun that goes with terrible is not 'terribility' but terror, and the noun that goes with comfortable is not 'comfortability' but comfort.