1.a.: to have a firm religious faith
b.: to accept something as true, genuine, or real
2.: to have a firm conviction as to the goodness, efficacy, or ability of something
3: to hold an opinion
Definition courtesy of Merriam-Webster Dictionary.
In the April 2015 General Conference of The Church of Jesus Christ of Latter-Day Saints, Elder L. Whitney Clayton gave a talk entitled "Choose to Believe". In it, he states:
"Belief and testimony and faith are not passive principles. They do not just happen to us. Belief is something we choose - we hope for it, we work for it, and we sacrifice for it. We will not accidentally come to believe in the Savior and His gospel any more than we will accidentally pray or pay tithing. We actively choose to believe, but like we choose to keep other commandments."
Believe is a verb.
A verb is an action.
Therefore, to believe is to ACT.