A chronic disease of the central nervous system manifested in convulsions or in impairment or loss of consciousness, and perhaps both. This disorder is linked with abnormal activity of the brain. A severely convulsive epileptic fit accompanied by unconsciousness is called grand mal, whereas the mild form, attacks of which are of very brief duration, is termed petit mal, these being two principal types of epilepsy. An epileptic is a person afflicted with epilepsy.

On the day following the transfiguration, Jesus Christ healed an epileptic that his disciples had been unable to cure. (Mt 17:14-20) From his childhood this boy had a “speechless and deaf spirit” that, among other things, periodically threw him into convulsions; these were accompanied by foaming at the mouth. Jesus rebuked the demon, it came out, and the boy was thus healed.​—Mr 9:14-29; Lu 9:37-43.

Though demon activity was associated with epileptic symptoms in this particular case, epilepsy normally has natural causes, and the Scriptures do not imply that it is generally caused by demon possession. Rather, Matthew (4:24) reports that people brought Jesus ailing ones including “demon-possessed and epileptic” persons, drawing a distinction between these two types of individuals cured by Christ.

The English term “epilepsy” is derived from the Greek word e·pi·le·psiʹa, meaning literally “seizure.” However, e·pi·le·psiʹa is not used in the Bible. Rather, for this disorder Matthew (4:24; 17:15) employed forms of the Greek word se·le·ni·aʹzo·mai, meaning, literally, “be moonstruck.” Whereas the King James Version uses “lunatick,” certain modern translations employ “epileptic(s)” at Matthew 4:24; 17:15.​—AS; NW; RS.

Interestingly, The International Standard Bible Encyclopaedia states: “The original meaning of the term seleniazomai, ‘moon-struck,’ is connected with the popular belief, widespread and of strange persistency, that the moon, in certain of  its phases, is injurious to human beings, esp[ecially] in the case of diseases of a periodic or remittent character. There are no data by which to determine whether, in the N[ew] T[estament] times, this particular word represented a living and active belief or had passed into the state of usage in which the original metaphor disappears, and the word simply indicates the fact signified without reference to the idea embodied in the etymology. We still use the word ‘lunatic’ to signify a person mentally diseased, although we have long since ceased to believe in the moon’s influence in such cases.”​—Edited by J. Orr, 1960, Vol. III, p. 1941.

Matthew’s use of forms of se·le·ni·aʹzo·mai does not mean that he held any superstitious views associating such a disease with certain phases of the moon. Evidently, he was merely employing the Greek term that was then commonly used to denote an epileptic. Also, the symptoms Matthew, Mark, and Luke describe as present in the boy’s case are certainly those associated with epilepsy.