Senones are just tied triphone HMM states. A context dependent HMM recognizer has a 3-5 state HMM for every context dependent phone. Conceptually, each different HMM state in each different phone HMM has its own Gaussian mixture model, but this is awful because many of them don't get much data assigned to them. So people share parameters for different HMM states based on a data driven decision tree that clusters states together. Those clustered or tied states are sometimes called senones.