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An ostensive definition conveys the meaning of a term by pointing out examples. This type of definition is often used where the term is difficult to define verbally, either because the words will not be understood (as with children and new speakers of a language) or because of the nature of the term (such as colors or sensations). It is usually accompanied with a gesture pointing out the object serving as an example, and for this reason is also often referred to as "definition by pointing." Ostensive definitions rely on an analogical or case-based reasoning by the subject they are intended to.^{[citation needed]}

For example, defining "red" by pointing out red objects -- apples, stop signs, roses -- is giving ostensive definition, as is naming. It is thought that children may learn a great deal of their language ostensively.^{[citation needed]}

Ostensive definition assumes the questioner has sufficient understanding to recognize the type of information being given. Ludwig Wittgenstein writes: So one might say: the ostensive definition explains the use--the meaning--of the word when the overall role of the word in language is clear. Thus if I know that someone means to explain a colour-word to me the ostensive definition "That is called 'sepia' " will help me to understand the word.... One has already to know (or be able to do) something in order to be capable of asking a thing's name. But what does one have to know?^[1]

The limitations of ostensive definition are exploited in a famous argument from the Philosophical Investigations (which deal primarily with the philosophy of language), the private language argument, in which Wittgenstein asks if it is possible to have a private language that no one else can understand.^[2]

The term ostension is also used by those who study folklore and urban legends to indicate real events that mirror or strongly resemble the events told in pre-existing and well-established legends and lore. Occasionally, a story that was originally fictitious is actually enacted at some later date, whereupon advocates of the original story may claim that it is true. But it may still be argued that the original version of the story is untrue, since it predates the event that can be cited as proof of it.^{[citation needed]}

A closely related term, pseudo-ostension, refers to fictional events that are intentionally acted out by persons aware of the original narrative. For example, the Fred Tuttle campaign in Vermont first appeared as a comic movie, and later was enacted (by the same people) as an actual political campaign.^{[citation needed]}

Neither term refers to events that appear to fulfill an existing prophecy, as the birth of Jesus or the arrival of Cortes in Mexico were later said to do. Instead, ostension and pseudo-ostension refer to claims about historical fact.^{[citation needed]}

1. ^ Wittgenstein, Ludwig. Philosophical Investigations, §30.
2. ^ Wittgenstein, Ludwig. Philosophical Investigations, §258.

Wittgenstein, Ludwig (1953/2001). Philosophical Investigations. Blackwell Publishing. ISBN 0-631-23127-7.