private language

Short definition from Wiktionary

Noun

1. 1
   Used other than figuratively or idiomatically: see private, language.
2. 2
   A language which expresses one's inner thoughts, feelings, or experiences but which cannot be used for communication, since it is known to and understandable by only one person—the existence of which was famously argued by Ludwig Wittgenstein (1889-1951) to be impossible.