A distance intentionally kept from something or between something, equal (literally or figuratively) to an arm's span. Typically used in the phrase "keep at an arm's length." Nancy has been keeping me at an arm's length ever since she found out I was gossiping about her. You're filthy from that game, please stay an arm's length away from me! You can watch me cook, but stay at arm's length in case any hot oil spurts out of the pan.