As a Noun: 1. The thought that something is not true, or false. English = disbelief. 2. A person who does not believe something; a person who has doubts about the truth of something. English = disbeliever, sceptic. 3. A person who does not believe that there is a God and/or does not believe in a religion. English = atheist, non-believer. 4. The belief that there is no God. English = atheism. As a Verb or Adjective: 1. To not think that something is true; to think that something is false. English = not believe, disbelieve. 2. To be not at all certain that something is true or right; to have doubts about something. English = (be) unconvinced, (be) sceptical. 3. To not believe that there is a God.