In my line of work, we convert words to numbers, and we have a codex that defines meanings for the numbers; that way, we can write publicly but (in making sure we choose words that, when converted to numbers, convert to numbers that have the second meaning we want) our public writing will only be understood on this second level with this second meaning only by those who use this system and codex. It's something like…
I believe the actual term in this case would be steganography.