Short for committed information rate, a specified amount of guaranteed bandwidth (measured in bits per second) on a Frame Relay service. Typically, when purchasing a Frame Relay service, a company can specify the CIR level they wish. The Frame Relay network vendor guarantees that frames not exceeding this level will be delivered. It's possible that additional traffic may also be delivered, but it's not guaranteed.
Some Frame Relay vendors offer inexpensive services with a CIR equal to zero. This essentially means that the network will deliver as many frames as it can, but it doesn't guarantee any bandwidth level.