Wheel
The wheel (cakkha) is a flat circular object that turns as it moves. To the early Buddhists the wheel was a very meaningful symbol.
The rim (mukhavaññi) represented movement or progress and the spokes (ara) and hub (nàbhi) represented the coming together of multiple things into a unity. Sympathy is to the world, the Buddha said, what the linchpin (àõã) is to the wheel (A.II,32), i.e. it keeps it functioning properly.
The Buddha's first discourse is called ‘Setting in Motion the Wheel of the Dhamma’ (S.V,420). A wheel flanked on either side by a deer has long been used to symbolize the Buddha’s teaching of this discourse at the Deer Park at Sarnath. Before the advent of statues, the Buddha was often represented by a wheel. What the cross is to Christians, the crescent and star to Muslims and the OM sign to Hindus, the wheel is to Buddhists.
References
The Buddhist Wheel Symbol, T. Karunaratne, 1969.