What is a blockchain?

A blockchain is a continuously growing list of records, called blocks, which are linked and secured using cryptography.
Harvard Business Review defines it as "an open, distributed ledger that can record transactions between two parties efficiently and in a verifiable and permanent way.

The first blockchain was conceptualised in 2008 by an anonymous person or group known as Satoshi Nakamoto and implemented in 2009 as a core component of bitcoin where it serves as the public ledger for all transactions.
The invention of the blockchain for bitcoin made it the first digital currency to solve the double spending problem without the need of a trusted authority or central server. The bitcoin design has been the inspiration for other applications.

Alternative blockchains, also known as altchains, are based on bitcoin technology in concept and/or code.
Altchains can provide solutions, including other digital currencies. Altchains target performance, anonymity, storage and applications such as smart contracts.

Distributed ledgers and other blockchain-inspired software are being developed by commercial organizations for various applications:
Deloitte and ConsenSys announced plans in 2016 to create a digital bank called Project ConsenSys.
R3 connects 80 of the world’s largest financial institutions to distributed ledgers built by Ethereum, Chain.com, Intel, IBM and Monax.
Visual Studio is making the Ethereum Solidity language available to application developers.
SafeShare Insurance offers blockchain-based insurance for the sharing economy, underwritten by Lloyd's of London.
A Swiss industry consortium, including Swisscom, the Zurich Cantonal Bank and the Swiss stock exchange, is prototyping over-the-counter asset trading on a blockchain-based Ethereum technology.
IBM offers a cloud Blockchain service based on the open source Hyperledger Fabric project.
Oracle Cloud offers Blockchain Cloud Service based on Hyperledger Fabric. Oracle has joined the Hyperledger consortium.
Hashgraph opens up exciting decentralized application use cases such as micro-payments, distributed capital markets, live collaboration apps, distributed MMOs, auctions

Notable blockchain application designs include:
Storj (pronounced: storage) aims to become a cloud storage platform that can’t be censored or monitored, or have downtime. It is the first decentralized, end-to-end encrypted cloud storage that uses blockchain technology and cryptography to secure your files.
LaZooz — decentralized real-time ride sharing
Steemit combines a blogging site/social networking website and a cryptocurrency
Hyperledger — cross-industry collaborative effort from the Linux Foundation to support blockchain-based distributed ledgers.
Counterparty — open source financial platform for creating peer-to-peer financial applications on the bitcoin blockchain
Bitnation is the world's first operational Decentralized Borderless Voluntary Nation, a Blockchain Powered Jurisdiction.
Ethereum is a Blockchain, with a Turing complete scripting language that enables the processing of smart-contracts on the Blockchain.
Level One Project from the Bill & Melinda Gates Foundation aims to use blockchain technology to help the two billion people worldwide who lack bank accounts

Skuchain technology is an end-to-end solution suite for the supply chain. Provides digitally permanent, auditable records that show stakeholders the state of the product at each value-added step. E.g: farm-to-shelf scenario.
Provenance is a platform that empowers brands to take steps toward greater transparency by tracing the origins and histories of products.
ShoBadge eliminates the need for usernames and passwords within the enterprise to authorize network and data access, instead using a blockchain-based digital identity verification and authentication process.
Stampd is a web application for the time stamping of documents on the decentralized public ledger of digital cryptocurrencies, commonly referred to as the blockchain. Notarizing your content on the blockchain is totally straight forward and cannot be forged by anyone.
With Slock.it, Airbnb apartments become fully automated, smart objects can be rented on demand and unused vehicles get a new lease on life. We're developing the future infrastructure of the Sharing Economy.
Synaps combines Symbiont’s leading smart contract technology and Ipreo’s new business process solution to help speed loan trade settlement.
Singapore-based CoinPip helps businesses to send money end-to-end to employees, freelancers and contractors around the world in under 48 hours, avoiding bank transfer fees and forex charges. Its goal is to reinvent how telegraphic transfers are made.
Abra is a bitcoin-based digital wallet app that lives on your smartphone. With Abra you can quickly and easily buy, sell, send, and store bitcoin.
Gyft Block is part of Gyft's broader mission to digitize gift cards so that they can be securely traded using bitcoin's public ledger, the blockchain.
Boardroom is a Governance Framework and dApp made for Individuals and Companies to manage their Smart Contract Systems on the Public and Permissioned Ethereum Blockchains.
Augur combines the magic of prediction markets with the power of a decentralized network to create a stunningly accurate forecasting tool - and the chance for real money trading profits.
OpenBazaar is a different way to do online commerce. It allows anyone to buy or sell anything they’d like to anybody else in the whole world, without anyone in the middle imposing rules or fees on them like eBay or Etsy do.

Everledger is a global startup assisting in the reduction of risk and fraud for banks, insurers and open marketplaces. Everledger uses blockchain to track features such as diamond cut and quality, as well as monitoring diamonds from war zones.

A group of healthcare companies, including the two pharmaceutical giants Genentech and Pfizer, have now announced the MediLedger project, which aims to revolutionize the management of pharmaceutical supply chains by using blockchain. The goal of the MediLedger project is to stop the circulation of stolen or counterfeit pills in the pharmaceutical supply chain and prevent them from getting into the hands of patients without knowing the origin of the drugs.