Blockchain sharding is a set of proposals for solving the Blockchain Scalability Trilemma. All these proposals share the idea of splitting the large blockchain database into smaller chunks that can be easily managed, and distributed between nodes in the network. This idea and the term "sharding" came from the theory of databases. Database sharding techniques are very useful for carrying out database scaling. No wonder blockchain sharding techniques are considered to be one of the most promising approaches to improve blockchain performance to process more transactions per second.
A fragment or partition of a whole blockchain database is often called a shard.
Sharding is a core idea of JaxNet protocol.
Classification of blockchain sharding proposals
First proposals for sharding Bitcoin network were discussed at the BitcoinTalk forum.