As you are looking for a word that represents a secret word (but not a language or a system), it would be codeword (or code word):
a word or phrase that has a secret meaning or that is used instead of another word or phrase to avoid speaking directly
From urbandictionary:
a word or phrase shared by a group which has a special meaning to that group
codewords are often used to hide meaning or intention from anyone not in the group
Also, there is cryptonym but it is usually used for names.
A word or name that is used secretly to refer to another; a code name or code word.
This question deserves a made-up word also: idiolexeme
Though, it is used as a nonce word in a very few sources one of which defines as an individual formation.
