Ostensive definition
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An ostensive definition conveys the meaning of a term by pointing out examples of what is defined by it. 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."
For example, defining "red" by pointing out red objects -- apples, stop signs, roses -- is giving ostensive definition, as is naming. Children learn a great deal of their language ostensively (as Wittgenstein claims in the Philosophical Investigations, though he makes the distinction that this is more "ostensive training" than actual definition, as the child is not yet capable of asking questions about language).
Ostensive definition tends to be imprecise, and not as useful when one does not already know the general nature of the term being defined; it assumes the questioner has sufficient knowledge to recognize the type of information being given. Wittgenstein also 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 he asks if it is possible to have a private language that no one else can understand.