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A reference, or a reference point, is the intensional use of one thing, a point of reference or reference state, to indicate something else. When reference is intended, what the reference points to is called the referent.
References are indicated by sounds (like onomatopoeia), pictures (like roadsigns), text (like bibliographies), indexes (by number) and objects (a wedding ring); but endless concrete and abstract methods can be used intentionally. This includes methods that intentionally hide the reference from some observers, as in cryptography.