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A Shard is a horizontal partition of data in a database or search engine. Each shard is held on a separate database server instance, to spread load.

Some data within a database remains present in all shards, but some appears only in a single shard. Each shard (or server) acts as the single source for this subset of data.

Source: https://en.wikipedia.org/wiki/Shard_(database_architecture)#cite_note-2