to have confidence in the truth, the existence, or the reliability of something, although without absolute proof that one is right in doing so

Only if one believes in something can one act purposefully.

to have confidence or faith in the truth of (a positive assertion, story, etc.); give credence to.

to have confidence in the assertions of (a person).

to have a conviction that (a person or thing) is, has been, or will be engaged in a given action or involved in a given situation

The fugitive is believed to be headed for the Mexican border.

to suppose or assume; understand (usually fol. by a noun clause)

I believe that he has left town.

believe in,

to be persuaded of the truth or existence of

to believe in Zoroastrianism; to believe in ghosts.

to have faith in the reliability, honesty, benevolence, etc., of

I can help only if you believe in me.

make believe. make ( def. 46 ) .

adjective

doubting , doubtful , mistrustful , untrusting , disbelieving , uncertain , unsure

adjective

presuming , assuming , accepting , having faith , positive , certain , convinced , sure , undoubting