invisible ink

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Ink that is colorless and invisible until treated by a chemical, heat, or special light.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An ink, invisible on application or soon thereafter, that can later be rendered visible by some process, such as exposing it to heat.