The term 'SECURE BY TYING DOWN' can be summarized by the word 'BELAY, ' which is a five-letter word. Belaying is a technique used in rock climbing and mountaineering to secure a climber to a fixed point, such as an anchor or a rope. It involves tying a rope around the climber's waist and attaching it to an anchor point, such as a carabiner or a BELAY device. This technique is used to ensure the safety of the climber and to prevent them from falling.