Another recent question on security margin included in its answer the discussion of how many rounds are broken so far. What is the definition of a round?
Many block ciphers are defined by specifying a round and then running that specification multiple times. For example, in AES, a round consists of the operations
SubBytes, ShiftRows, MixColumns, AddRoundKey. That is one round and, to get AES, you run that multiple times (plus some setup and some post-processing).
Thus a round is defined by each cipher and typically consists of a number of building blocks that are composed together to create a function that is run multiple times.