The term that most accurately describes parmigianino's "madonna of the long neck" is

The term is mannerism (Mannerist style).

Parmigianino is an Italian painter and engraver of the Renaissance and early Mannerism. It was one of the most brilliant painters of the 16th century.

The term Mannerism is commonly used today to refer mainly to certain artistic events carried out in Europe between about 1520 and 1620.