Full Idea
Ostensive definitions explain what an expression means by pointing to an object, action, event, etc. denoted by the expression.
Gist of Idea
Ostensive definitions point to an object which an expression denotes
Source
Thomas Mautner (Penguin Dictionary of Philosophy [1996], 'definition')
Book Reference
Mautner,Thomas: 'Dictionary of Philosophy' [Penguin 1997], p.126
A Reaction
These will need some context. If I define 'red' simply by pointing to a red square, you might conclude that 'red' means square. If I point to five varied red objects, you have to do the work of spotting the common ingredient. I can't mention 'colour'.