What does it mean to "believe" (John 3:16-18)?

John 3:18 
The one who believes in Him [Jesus] is not condemned. 
The one who does not believe has been condemned already, because he has not believed in the name of the one and only Son of God.

John 3:16 - 21

AMP - ¹⁶ For God so greatly loved and dearly prized the world that He [even] gave up His only begotten (unique) Son, so that whoever believes in (trusts in, clings to, relies on) Him shall not perish (come to destruction, be lost) but have eternal (everlasting) life. ¹⁷ For God did not send the Son into the world in order to judge (to reject, to condemn, to pass sentence on) the world, but that the world might find salvation and be made safe and sound through Him.

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