I hear this term all the time in war movies and such. I assume it has something to do with a coordinate system, perhaps minutes or seconds on a map, but am curious to know how far of a walk each ‘click’ is. Does it change from map to map or military to military? I know this is basic information out there on the net, but you can imagine what happened when I put in ‘click’ in with my search terms.
One click = one kilometer.
…and it seems to have become popular during the Viet Nam War. It first appears in print in 1962.
Just don’t tell me they used it in WWII along with “the whole nine yards.” 
Wow… two minutes- and with a link, no less. Thanks!
You might get better search results if you spell it “klick”, by the way.
Heinlein uses it in his stories a lot. I was clueless, until one character did a landing calculation in his head, and mumbled something about “10[sup]5[/sup] centimeters to the klick…”