"Polymorphism" means the ability to take several forms. In object-oriented programming, this refers to the ability of an entity to refer at run-time to instances of various classes. In a typed environment such as Eiffel, this is constrained by inheritance: ...
[The Author notes Meyer has a following section 10.1.7 on Static Type, dynamic type, which is relevant, but claims "... there is no way the type of an object can ever change. Only a reference can be polymorphic: ...". Meyer is clear between the concept and the Eiffel realization in his polymorphism definition above, but here neglects the "becomes" facility as found in several dynamically typed OO languages such as Actors, CLOS, Self and Smalltalk, which allows an object (and not just a reference) to change its class.]
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