Reference is a relation between objects in which one object designates, or acts as a means by which to connect to or link to, another object. The first object in this relation is said to refer to the second object. The second object, the one to which the first object refers, is called the referent of the first object.
References can take on many forms, including: a thought, a sensory perception that is audible (onomatopoeia), visual (text), olfactory, or tactile, emotional state, relationship with other, spacetime coordinate, symbolic or alpha-numeric, a physical object or an energy projection. In some cases, methods are used that intentionally hide the reference from some observers, as in cryptography.
References feature in many spheres of human activity and knowledge, and the term adopts shades of meaning particular to the contexts in which it is used. Some of them are described in the sections below.