It’s important to keep a particular audience in mind when explaining this technology. The explanation for the blockchain that I give a software developer or engineer is different than the explanation I give to non-technical people. For instant, this is the definition I gave in my book, for which the audience is the bit-curious layperson:
The most important and fundamental tool of a bank is a ledger, a written record of transactions and balances for each account held at the bank. Without this ledger, it would be difficult if not impossible to keep track of the ownership of assets and liabilities of both the bank’s customers and the bank itself. Similarly, a ledger is also fundamental to bitcoin. This ledger is called a “blockchain” and stores a complete record of every transaction that has been accepted by the bitcoin network since the launch of the “genesis block” at the beginning of bitcoin’s existence. The blockchain is stored and maintained by bitcoin “full nodes,” which are required for broadcasting and relaying transactions on the bitcoin network.
While the legacy banking system relies on “correspondents” and other intermediaries to move funds between banks when there is no direct connection, bitcoin instead relies on a system of peer-to-peer connections between full nodes to relay transactions to miners, and then relay blocks from miners back to the rest of the bitcoin network. After a block has been accepted as valid by a majority of nodes on the network, it will be added to the blockchain, and miners will then begin building the next block with the latest batch of transactions. New entrants to the bitcoin network will only accept the longest blockchain as valid, making it very difficult for someone to trick new nodes into downloading an inaccurate blockchain.
The book goes on to further explain mining and how blocks are added to the network in the next chapter. This explanation could be simplified even further, but you start to lose context and vital information which helps explain the “why” and “how” of blockchain technology.