Polymorph

Polymorphing was a magical effect in which the subject creature assumed a new physical form while retaining its essential identity and abilities. These effects included the Alter self spell, polymorph spells (polymorph, baleful polymorph, and polymorph any object), the shapechange spell, the wildshape class feature, and the alternate form special quality. Magic could cause creatures and characters to change their shapes&mdash;sometimes against their will but usually to gain an advantage. Polymorphed creatures retained their own minds but had new physical forms.

The polymorph spell defined the general polymorph effect.

Realmslore
In common speech polymorphing was sometimes known as a "spell-guise".