addict Definition

1. Someone who is physically or psychologically dependent on the habitual intake of a drug such as alcohol, nicotine, heroin, etc.

Thesaurus: user, slang: junkie, dope fiend, pothead, head, acid head.
2. colloq
Someone who is extremely fond of a hobby, etc.
Example: a chess addict
Thesaurus: enthusiast, buff (slang), fan, follower, devotee, slang: nut, freak.