As a Noun:
 1. The scientific study of the human mind and the reasons for people's behaviour. English = psychology.
 2. 
 3. A person who studies the human mind and tries to explain why people behave in the way that they do. English = psychologist.
 4. 
As a Verb or Adjective:
 1. Referring to people's mind and thoughts; relating to psychology. English = (be) psychological.