From a historical perspective, the term Named Entity was coined during the MUC-6 evaluation campaign[1] and contained ENAMEX (entity name expressions e.g. persons, locations and organizations) and NUMEX (numerical expression).
A more formal definition can be derived from the rigid designator by Saul Kripke.
As an example, consider the sentence, "Biden is the president of the United States".
Rigid designators usually include proper names as well as certain natural terms like biological species and substances.
There is also a general agreement in the Named Entity Recognition community to consider temporal and numerical expressions as named entities, such as amounts of money and other types of units, which may violate the rigid designator perspective.