Polymorph

Polymorphing is a magical effect in which the subject creature assumes a new physical form while retaining its essential identity and abilities. These effects include the Alter self spell, polymorph spells (Polymorph(spell), Baleful polymorph, and Polymorph any object), the Shapechange spell, the wildshape class feature, and the alternate form special quality. Magic can cause creatures and characters to change their shapes—sometimes against their will, but usually to gain an advantage. Polymorphed creatures retain their own minds but have new physical forms.

The polymorph(spell) spell defines the general polymorph effect.

Realmslore
In common speech polymorphing is known as a spell shape, a spell-guise or spell-disguise