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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.
For example, defining "red" by pointing out red objects—apples, stop signs, roses—is giving ostensive definition, as is naming.
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.
John Passmore tells us that the term was first defined by the British logician William Ernest Johnson (1858-1931):
"His neologisms, as rarely happens, have won wide acceptance: such phrases as “ostensive definition”, such contrasts as those between ... “determinates” and “determinables”, “continuants” and “occurrents”, are now familiar in philosophical literature" (Passmore 1966, p. 344).
Ostension in folklore
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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. Semiotician Umberto Eco was the first to use the term to describe the way in which people communicate messages 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 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.":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 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. Folklorist John McDowell, in an article preceding Dégh and Vázsonyi by a year, explored the relationship between iconicity—representation—and ostension—presentation—in mythic narrative, finding in episodes of ostention a vitual encounter with the experiential substrate, an experience that he termed "narrative epiphany." 
- Enumerative definition
- Extensional definition
- Intensional definition
- Legend tripping
- Wittgenstein, Ludwig. Philosophical Investigations, §30.
- Wittgenstein, Ludwig. Philosophical Investigations, §258.
- Eco, Umberto. A Theory of Semiotics. 224-26.
- Dégh, Linda; Vázsonyi, Andrew (1983). "Does the Word 'Dog' Bite? Ostensive Action: A Means of Legend Telling". Journal of Folklore Research 20: 5–34.
- Ellis, Bill. Legend-Trips and Satanism: Adolescents' Ostensive Traditions as "Cult" Activity. In Richardson et al., The Satanism Scare, pp. 279-295.
- FOAFTale News 25 (March 1992): 11.
- Goldstein, Diane. Once upon a Virus (2004).
- "Beyond Iconicity: Ostension in Kamsa Mythic Narrative, Journal of the Folklore Institute 19 (1982): 119-139.
- Passmore, John (1966). A Hundred Years of Philosophy (2nd ed.). London: Penguin (1957).
- Dégh, Linda; Andrew Vázsonyi (1983). "Does the Word 'Dog' Bite? Ostensive Action: A Means of Legend Telling". Journal of Folklore Research 20: 5–34.
- Eco, Umberto (1976). A Theory of Semiotics. Indiana University Press. ISBN 0-253-20217-5.
- Goldstein, Diane E (2004). Once upon a Virus: Aids Legends and Vernacular Risk Perception. Logan, UT: Utah State University Press. ISBN 0-87421-587-0.
- Richardson, James T.; Best, Joel; Bromley, David (1991). "The Satanism Scare". New York: Aldine de Gruyter. ISBN 0-202-30379-9.
- Wittgenstein, Ludwig (1953/2001). Philosophical Investigations. Blackwell Publishing. ISBN 0-631-23127-7. Check date values in:
- William Van Orman Quine (1974). The Roots of Reference. La Salle, Illinois: Open Court Publishing Co. (in particular Sect.11)