A polymorphic data field is one in which the values may be of different natures and/or units corresponding to the underlying data having more than one possible representation.
For example, a specific event in someone's life could be measured either by the Calendar Date or by their Age. If both sorts of values were stored in the same field then the data does not have a single representation and is labelled Polymorphic.
Generally, polymorphic data only occurs in source fields corresponding to unprocessed information. In such cases, derived fields have been created to give a purer representation of the information.