RUCK•ING [VERB] carrying a weighted pack on your back. It implies action, energy, and purpose.
Rucking is the foundation of Special Forces training. Green Berets are well versed in shouldering heavy rucks through cities, mountains, jungles and deserts, in war and in peace — alone when they must, together whenever possible. Rucking requires strength, endurance, and character — and builds it, too.
Note: Hiking is rucking in the mountains, urban hiking is simply called rucking. Rucking can be done at any weight.
The above is taken form the masters of endurance rucking events, GORUCK. Check out the What is Rucking video here.