"A letter, perhaps, written in invisible ink?"
-- Severus Snape (PA14)
Invisible Ink vanishes as it dries. It is used to keep writing secret from prying eyes.
Wizards doing magical research may try to hide their work from their rivals by using invisible ink. This can be a problem if they have forgotten what piece of parchment they were using. A Revealing Charm will, however, expose magical invisible ink to view (BoS).
Hermione Granger tried to use the Revealing Charm on Tom Riddle’s Diary, in case it had been written with invisible ink (CS13). Severus Snape speculated whether the confiscated Marauder’s Map might be a letter written in invisible ink that Harry Potter wanted to hide (PA14).
Invisible ink is mentioned by the Roman writer Tacitus in 4th century BC (Wikipedia).
Tincture of Demiguise is invisible to the naked eye. Albus Potter and Scorpius Malfoy use it to write a message on a blanket (CC4.5). It may provide an ingredient for the invisibility element of this ink.