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A blockchain is defined as a P2P¹ distributed ledger² forged by consensus³.
In essence, a blockchain is a distributed database built to log, store and share secure, verified and unmodifiable data.
P2P networking is a distributed application architecture that partitions tasks or workloads between peers.
A P2P network is designed around the notion of equal peer nodes simultaneously functioning as both "clients" and "servers" to the other nodes on the network. This model of network arrangement differs from the client–server model where communication is usually to and from a centralized server.
The decentralized nature of P2P networks increases robustness because it removes the single point of failure that can be inherent in a client-server based system. As nodes arrive and demand on the system increases, the total capacity of the system also increases, and the likelihood of failure decreases.
If one peer on the network fails to function properly, the whole network is not compromised or damaged. In contrast, in a typical client–server architecture, clients share only their demands with the system, but not their resources. In this case, as more clients join the system, fewer resources are available to serve each client, and if the central server fails, the entire network is taken down.
In short, it is a system of nodes (computers) powering the entire network acting as both the supplier and consumer of resources, with decentralization as a key feature.
A ledger is a system containing all the records of a input and output of a process, in the case of cryptocurrencies: transactions.
A distributed ledger is a data structure which is spread across different computing devices.
The purpose of a consensus mechanism is to bring all the nodes in agreement on a single version of the blockchain.