An ink which leaves no visible mark when used on paper (or other, perhaps treated, surface). The intended recipient must perform some action in order to make the message visible.

Technically, this is a form of steganography; however, it also has a physical aspect (is it really that hard to detect the ink even if invisible?).

You can write with a toothpick dipped in lemon juice; this leaves almost no marks once dry, but the letters can be read by heating the paper over an exposed flame (the marks appear brown; apparently something just gets burnt). If using behind enemy lines, be sure to have a ready explanation for the large amount of lemons and toothpicks carried with you.