secret ink

noun

Definition of secret ink

:a fluid for invisible writing to be made visible afterwards: such as
a :a colorless plant juice that can be made visible by subjecting to heat
b :one of a pair of water solutions of chemicals that are colorless or nearly so and that when combined form a strongly colored compound
c :a water solution of a fluorescent substance that is colorless in visible light but leaves characters on paper that fluoresce when exposed to ultraviolet light

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