WORKING DEFINITION
A block contains an ordered set of transactions. It is cryptographically linked to the preceding block, and in turn it is linked to be subsequent blocks. The first block in such a chain of blocks is called the genesis block. Blocks are created by the ordering service, and then validated and committed by peers.
https://hyperledger-fabric.readthedocs.io/en/latest/glossary.html
A block contains one or more transactions. The contents of the block are not encrypted in the blockchain. A block, in general, contains valid and invalid transactions. However invalid transactions have no effect on the State.
A block usually contains three sections: a blockheader, the payload (with at least the transactions) and the metadata section (containing the valid/invalid indicator per transaction).
Fabric: A block in Fabric contains both valid and invalid transactions.
Sawtooth: A block in Sawtooth contains only valid transactions.
Burrow: A block in Burrow contains only valid transactions.
Iroha: A block in Iroha contains only valid transactions.
Block - A set of transactions that are bundled together and added to the chain at the same time.