Language game - Language games are examples of language use far simpler than our own. Wittgenstein uses language games to highlight certain features of language. For instance, a language game that includes only the names of building objects and the words for numbers highlights the sharp difference between these two kinds of words. In the Blue Book, Wittgenstein conceives of language games as more primitive forms of language that can be built upon gradually in order to achieve the complexity of ordinary language. In the Brown Book, Wittgenstein acknowledges that language games are distinct languages, and that the words used in these simpler languages do not have the same meaning as their equivalents in our more complex language.
Notation - In the Blue Book, Wittgenstein uses the concept of notation almost synonymously with the concept of language games. He suggests that metaphysical assertions rarely consist in making new discoveries, but are simply appeals for a different notation. For instance, if someone says "only what I see is really seen," that person might think he is voicing a metaphysical discovery. The only way those words make sense, however, is if we understand them as appealing for a redefinition of the words "really seen" so that they refer to only what that person sees. That person is appealing for a different notation: he wants the way we use words to be different from the way they are currently used.
Ostensive definition - To define a word ostensively is to define a word by pointing to its literal counterpart. For instance, I can give an ostensive definition of a chair by pointing to a chair and saying, "this is a chair." Such definition is opposed to dictionary-style definitions that use only words to explain the meaning of other words. Ostensive definition gives us a sense that we are making a real connection between language and the world. We might be tempted to take ostensive definition as the paradigmatic form of definition and the kinds of objects that can be so named as the paradigmatic form of words.
Solipsism - The view that "only I exist." If everything I experience is nothing more than the report of my senses to my brain, I have no reason to believe that anything outside my mind exists. If I were to stop sensing, these things would cease to exist for me. Solipsism is the philosophical position that denies the existence of an external world.