Question

define right hand thumb rule.

Answer 1

According to right hand thumb rule if we place our hand in such a way that fingers will represent the magnetic field created by the current and thumb will represent the direction of current

Answer 2

The direction in vector cross product is determined by the right hand screw rule and the right hand thumb rule.

For example: When A×B=C, then the direction of C is right angles to the plane having vectors A and B.