From a technical point of view, the most fundamental definition of a transaction is an atomic event that is allowed by the underlying protocol. In a blockchain, it is a event that is broadcast to the network and collected into blocks. In the case of the Steem blockchain, transactions include:
Each transaction are indexed with a transaction hex that is unique and allows anyone to prove that a particular event has happened. Steemd.com displays a record of every single transaction that ever happened on the blockchain.