Invisible Boundary That Is Not to Be Crossed

This page is the CodyCross answer for the clue below from Space Exploration, Group 913, Puzzle 5 (Andromeda Galaxy).

Question is: Invisible Boundary That Is Not to Be Crossed

and the answer is: REDLINE

Word Meaning:

RED LINE — (ice hockey) The thick red line on the ice which divides the rink in half.

Definitions from Wiktionary.