To change from one form to another form. Can be done both voluntarily and involuntary.
Polymorph means to change the shape or form of a creature or object. It can be used as a means to buff or otherwise aid a character, or be used to harm or debilitate an enemy. An example of the former would be transforming into a fire elemental to safely traverse lava, whereas an example of the latter would be turning an enemy into a newt.