Actually, you need to go to the real definition and ask Mr. Webster.
pis·tol
[pis-tl] Show IPA noun, verb, pis·toled, pis·tol·ing or ( especially British ) pis·tolled, pis·tol·ling.
noun
1.
a short firearm intended to be held and fired with one hand.
re·volv·er
[ri-vol-ver] Show IPA
noun
1.
a handgun having a revolving chambered cylinder for holding a number of cartridges, which may be discharged in succession without reloading
So, pistol covers both. Revolver only covers them, as a semi-automatic covers those. There are also single-shot pistols.
The real term would be handgun
hand·gun
[hand-guhn] Show IPA
noun
any firearm that can be held and fired with one hand; a revolver or a pistol.