Kind of Ink That Spies Might Use
This page is the CodyCross answer for the clue below from The 80s, Group 791, Puzzle 4 (Space Exploration).
Question is: Kind of Ink That Spies Might Use
and the answer is: INVISIBLE
Word Meaning:
INVISIBLE — Unable to be seen; out of sight; not visible.
Definitions from Wiktionary.