Fool, idiot, imbecile, moron, simpleton, natural are often used popularly and interchangeably of one regarded as lacking sense or good judgment but each can be more precisely applied to someone mentally deficient in a given degree.
Fool, the most general, can apply to anyone mentally deranged as well as mentally deficient, implying lack or loss of reason or intelligence; it may be used as an extremely offensive term of contempt.
Idiot, imbecile, and moron are technical designations for one mentally deficient.
An idiot is incapable of connected speech or of avoiding the common dangers of life and needs constant attendance.
An imbecile is incapable of earning a living but can be educated to attend to simple wants or avoid most ordinary dangers.
A moron can learn a simple trade but requires constant supervision in his work or recreation. In more general and nontechnical use idiot implies utter feeblemindedness, imbecile implies half-wittedness, and moron implies general stupidity.
All three, however, may imply no more than often mild derogation or disapprobation of a person or his conduct.
Simpleton, a term of indulgent contempt, implies silliness or lack of sophistication.
Natural, which persists chiefly in historical context, may designate any congenitally feebleminded person.