Referent

A referent is a term that is ambiguous in that it can mean the subject, the person or thing which refers to something else, or the object, the thing which is referred to. However in specific fields, science, sociology, linguistics, grammar, this ambiguity may be clarified by specifying that in that field the referent is always the subject, or always the object.

See also Wiktionary:referent and Wikipedia article reference.