secret ink
nounDefinition 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