3.343 Definitions are rules for translation from one language into another. Every right sign-language must allow of translation into every other by means of such rules: This is what they must all have in common.
There is no ostensive definition in this sense. To define is to determine one bit of language as equivalent to another. Any adequate language will be able to cope with all such possible translations (perhaps just by having a means for importing foreign words and phrases, so that the English for Brie is “Brie” for instance). Again, what is had in common by bits of language is just this relation between them, not anything metaphysical.