Secret writing

Definition
Secret writing is a

"technique employing invisible messages hidden in or on innocuous materials. This includes invisible inks and microdots, among many other variations."

Overview
"The simplest secret writing uses organic inks: milk, vinegar, lemon juice, even urine. These inks dry invisibly and can be developed by applying heat. Espionage agencies have produced many inks made of chemicals that could be developed only by a specific chemical."