Blockchain transactions are the backbone of cryptocurrencies. Blockchain transactions function in a number of different ways and they bring huge advantages in terms of transactional speed and transfer fees.
The blockchain is a database; a network of computers that stores transactional data in replica across every PC (node) in the system. This data is called a distributed ledger. The data is entered into the chain in intervals known as blocks, and the blocks form a chain to form a block-chain. Each block is time-stamped and its order and transactions verified.
How to define a blockchain transaction depends on which blockchain you are talking about. Ultimately, a transaction is a single event that is permitted by the underlying protocol.