A transaction is one complete transfer of data over the bus, while a bus cycle is the time it takes for the bus clock to go high, then go low and then start to go high again. Almost all buses are set up so that transactions take two time cycles in order to allow enough time and enough high/low transitions to occur for the values to be latched into interface registers and for the wires to go back to a quiescent state before the next transaction. In the world of buses, it is not a good idea to shove electronic events too close together in time.