microdot

/ˈmaɪkroʊˌdɑt/
noun
  1. A very small photograph or piece of text, often used for secret communication or espionage.
    • The spy hid a microdot inside a book binding.
    • He used a microscope to read the microdot on the letter.
    • Microdots were a common tool for covert messages during the Cold War.
  2. A tiny pill or tablet, especially one containing a psychedelic drug like LSD.
    • In the 1960s, microdots were a popular form of LSD.
    • The microdot was so small it could be placed under the tongue.
    • He found a microdot in the bottom of the bag.