What are rounds in the context of ciphers? 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.