Longman Dictionary of Contemporary English
The Collaborative International Dictionary
invisible ink \in*vis"i*ble ink`\, n. A fluid that has no color in the visible spectrum, but may be detected under certain conditions, as under ultraviolet light. It may be used to write notes not readable under normal light.
Wiktionary
n. An ink, invisible on application or soon thereafter, that can later be rendered visible by some process, such as exposing it to heat.
Wikipedia
Invisible ink, also known as security ink, is a substance used for writing, which is invisible either on application or soon thereafter, and can later be made visible by some means. Invisible ink is one form of steganography.
Usage examples of "invisible ink".
I was surprised that Alethea with her obession with secrecy had not insisted upon a code or an invisible ink.
So I think we can add a long list of lesser names below theirs, in invisible ink, hm?
I sometimes felt as if these marks on my body were a kind of code, which blossomed, then faded, like invisible ink held to a candle.
Next she exposed the pages one by one to heat, in case there should be writing in invisible ink between the lines.
Why else would he have kept a secret journal, not only in code, but written in an invisible ink?
Countless tables were strewn with crisp rolls of parchment, bright purple quill pens, and crystal pots of invisible ink.