addictive

Short definition from Wiktionary

Adjective

1. 1
   Causing or tending to cause addiction; habit-forming.
2. 2
   Enjoyable, so that one comes back for more.
3. 3
   Characterized by or susceptible to addiction.

Noun

1. 1
   A drug that causes an addiction.
2. 2
   Anything that is very habit-forming.