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.
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?
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.
The term ostension is also used by those who study folklore and urban legends to indicate real-life happenings that parallel the events told in pre-existing and well-established legends and lore. The term was first used by semiotician Umberto Eco to describe the way in which people communicate message through miming actions, as by holding up a pack of cigarettes to say, "Would you like one?" The concept was applied to contemporary legends by folklorists Linda Dégh and Andrew Vázsonyi, who defined it as the way in which "the reality itself, the thing, the situation or event itself functions in the role of message" Dégh and Vázsonyi argued that the most direct form of ostension involved committing an actual crime mentioned in a well-known urban legend, such as microwaving someone's pet animal or placing poison in a child's Halloween candy. While such events are rare, the authors stressed that folklorists must recognize "that fact can become narrative and narrative can become fact" (p. 29).
Dégh and Vázsonyi, followed by other analysts, argued that there were two other forms of ostension that did not necessarily involve literal acting out of legends. Quasi-ostension involves interpretation of ambiguous events in terms of a legend, as when a murder is first believed to have been a "cult" sacrifice or "gang" murder when in fact the perpetrator had other motives. Many local media panics are based in this form of ostension.
Pseudo-ostension involves legend-like events that intentionally acted out by persons aware of the original narrative. For example, in 1991, Ebony, published a letter written by "C.J." a Dallas-area woman who said she was HIV-positive, but intentionally having sex with as many men as possible. Soon after, a local radio talk-show broadcast a phone call from a woman who said she was the real "C.J." "I blame it on men, period" she said to the talk-show host. "I'm doing it to all the men because it was a man that gave it to me." After a huge spike in males seeking HIV screening in the Dallas-Fort-Worth area, both the author of the letter and the talk-show caller were identified as hoaxers intending to raise consciousness of the disease.
Ostension has become an important concept for folklorists studying the ways in which folklore affects everyday people's real lives, ranging from supernatural rituals such as Legend tripping to the complex ways in which awareness of AIDS has affected people's sexual habits.