Blockchain

At its core, blockchain is a system for recording information about a transaction in a new decentralized way that makes it difficult or impossible to alter. These transactions are stored on sheets or blocks in a digital ledger that is shared among the participants of the network. Consensus on the transactions bring the peer-to-peer network into agreement. Once the agreed upon transactions blocks are recorded in the immutable ledger, trust becomes a fundamental component built into the system.  BJM

According to hyperledger.org,

"A blockchain is a peer-to-peer distributed ledger forged by consensus, combined with a system for "smart contracts" and other assistive technologies".

Smart contracts are simply computer programs that execute predefined actions when certain conditions within the system are met.

Consensus refers to a system of ensuring that parties agree to a certain state of the system as the true state.

Sawtooth: Distributed ledger that records transactions, in chronological order, shared by all participants in a Sawtooth network. Each block on the blockchain is linked by a cryptographic hash to the previous block. https://sawtooth.hyperledger.org/docs/core/releases/latest/glossary.html

A blockchain is a chain of blocks each containing transaction (transition) data. Each block, except the first block, is linked with the previous block and each block, except the last block, is linked with the next block, together forming a chain.

Once a block has entered the blockchain, it is ‘chiselled in granite’. This characteristic delivers the immutability of the data in the blockchain, also referred to as practically tamper-resistant data or virtually incorruptible data. This aspect is one of the main reasons for the broad interest in blockchain technology.

[For those with software development experience: In computer science language a blockchain is an append-only data structure; a blockchain (instance) consists at any moment in time of a number of blocks. If the chain has N blocks, then it has N-1 links, valid for N>=1. The blockchain contains all the transitions, while the World State is derived from all transitions (there is a better optimization as World State (N) = valid transactions in block N applied to World State (N-1).]