Crossword clues for the word: LIGHTSPEED The word LIGHTSPEED has appeared as an answer to at least 4 crossword clues on different crossword puzzles. Crossword clues for LIGHTSPEED Showing 1 to 4 of 4 results Clue(s) "186,000 miles per second" "What ""c"" might mean" Special relativity limit Velocity that's impossible to attain