Moon letter is a term used in old Norse literature to refer to a hidden or secret message revealed only in certain lighting conditions. Synonyms for this intriguing term include "cipher script", which alludes to the encrypted nature of the letter. Another synonym is "lunar epistle", highlighting the association with the moon's phases. Similar terms such as "occult missive" or "cryptic dispatch" also convey the secretive nature of the message. Additionally, the phrase "night-time communiqué" can be used to describe a moon letter, emphasizing its delivery under the cover of darkness. Each synonym beautifully encapsulates the mystique and enigma surrounding the concept of a moon letter.