Blockchain is a distributed database with a list (that is, chain) of records (that is, blocks) linked and secured by digital fingerprints (that is, cryptho hashes).
Blockchain is a continuously growing list of records, called blocks, which are linked and secured using cryptography. Each block typically contains a hash pointer as a link to a previous block, a timestamp and transaction data. By design, blockchains are inherently resistant to modification of the data. A blockchain can serve as "an open, distributed ledger that can record transactions between two parties efficiently and in a verifiable and permanent way."
[Blockchain is ...] a solution to the double-spending problem using a peer-to-peer network. The network timestamps transactions by hashing them into an ongoing chain of hash-based proof-of-work, forming a record that cannot be changed without redoing the proof-of-work. The longest chain not only serves as proof of the sequence of events witnessed, but proof that it came from the largest pool of CPU power... The network itself requires minimal structure.
-- Satoshi Nakamoto @ Bitcoin Whitepaper
Blockchain [data structure] is an ordered, back-linked list of blocks of transactions. The blockchain can be stored as a flat file, or in a simple database. The Bitcoin Core client stores the blockchain metadata using Google's LevelDB database. Blocks are linked "back," each referring to the previous block in the chain. The blockchain is often visualized as a vertical stack, with blocks layered on top of each other and the first block serving as the foundation of the stack. The visualization of blocks stacked on top of each other results in the use of terms such as "height" to refer to the distance from the first block, and "top" or "tip" to refer to the most recently added block.
Each block within the blockchain is identified by a hash, generated using the SHA256 cryptographic hash algorithm on the header of the block. Each block also references a previous block, known as the parent block, through the "previous block hash" field in the block header. In other words, each block contains the hash of its parent inside its own header. The sequence of hashes linking each block to its parent creates a chain going back all the way to the first block ever created, known as the genesis block.
-- Andreas M. Antonopoulos @ Mastering Bitcoin: Programming the Open Blockchain
Blockchain is a public database where new data are stored in a container called a block and are added to an immutable chain (hence blockchain) with data added in the past. In the case of Bitcoin and other cryptocurrencies, these data are groups of transactions. But, the data can be of any type, of course.
Blockchain is a public database that consists out of blocks that anyone can read. Nothing special, but they have an interesting property: they are immutable. Once a block has been added to the chain, it cannot be changed anymore without invalidating the rest of the chain.
-- Xavier Decuyper @ Writing a Tiny Blockchain in JavaScript
Blockchain is a distributed database with a set of rules for verifying new additions to the database.
-- Eric Munsing @ Build Your Own Blockchain: A Python Tutorial
Blockchain is a digital ledger in which transactions made in bitcoin or another cryptocurrency are recorded chronologically and publicly.
Blockchain is an incorruptible digital ledger of economic transactions that can be programmed to record not just financial transactions but virtually everything of value.
-- Don & Alex Tapscott, Blockchain Revolution (2016)
Blockchain is a data structure that makes it possible to create a digital ledger of data and share it among a network of independent parties.
-- Tiana Laurence @ Blockchain for Dummies (2017)
Blockchain is a shared, distributed ledger that facilitates the process of recording transactions and tracking assets in a business network. An asset can be tangible - a house, a car, cash, land - or intangible like intellectual property, such as patents, copyrights, or branding. Virtually anything of value can be tracked or traded on a blockchain network, reducing risk and cutting costs for all involved.
-- Manav Gupta @ Blockchain for Dummies - IBM Limited Edition (2017)
Blockchain is a shared, immutable ledger for recording the history of transactions. It fosters a new generation of transactional applications that establish trust, accountability and transparency - from contracts to deeds to payments.
Blockchain is the magic that will power all the world's financial transactions and unlock the great decentralized database in the sky.
-- Snake Oil Purveyors
Blockchain is a provably-immutable append-only data log with transaction validation based on asymmetric crypto, and (optionally) a Byzantine-generals solution too!
Blockchain is really a fairly old set of ideas - merkle hash trees, distributed databases etc. The novelty, if any, is in a convergence of different methods and new levels of efficiency. This is the stuff of solid, and valuable, progress. But it isn't a revolution in governance, production or the structure of society.