This book is an ontological introduction of blockchain that clarifies the current confusion and misconceptions on the subject.  It discusses the theoretical foundation of the various technology components commonly used to compose blockchain implementations. Blockchain increasingly finds application in multiple industries from banking and fintech, logistics to media-publishing.  It has been frequently claimed that blockchain, just like TCP/IP to the Internet, can form the fabric over which various applications (especially, those for banking, trade and finance) can be built. The author proposes a unifying definition for blockchain based on the common characteristics of known constructions, namely, a peer-to-peer network, a hash chain, a consensus protocol and a programmable/smart contract functionality. This book serves as a resource for students, developers and managers who have little knowledge about blockchain but wish to understand the technology or use it for their applications.