Easton's 1897 Bible Dictionary:

Daemoniac
   one "possessed with a devil." In the days of our Lord and his
   apostles, evil spirits, "daemons," were mysteriously permitted
   by God to exercise an influence both over the souls and bodies
   of men, inflicting dumbness (Matt. 9:32), blindness (12:22),
   epilepsy (Mark 9:17-27), insanity (Matt. 8:28; Mark 5:1-5).
   Daemoniacs are frequently distinguished from those who are
   afflicted with ordinary bodily maladies (Mark 1:32; 16:17, 18;
   Luke 6:17, 18). The daemons speak in their own persons (Matt.
   8:29; Mark 1:23, 24; 5:7). This influence is clearly
   distinguished from the ordinary power of corruption and of
   temptation over men. In the daemoniac his personality seems to
   be destroyed, and his actions, words, and even thoughts to be
   overborne by the evil spirit (Mark, l.c.; Acts 19:15).