microdot

Short definition from Wiktionary

Noun

1. 1
   A text or photographic image that has been reduced in size to that of a typographical dot in order to escape detection by unintended recipients.
2. 2
   A small-sized tablet containing lysergic acid diethylamide (LSD).

Verb

1. 1
   To mark with a microdot.
2. 2
   To make into a microdot.