A database shard is a horizontal partition in a database or search engine. Each individual partition is referred to as a shard or database shard.
Horizontal partitioning is a database design principle whereby rows of a database table are held separately, rather than being split into columns (which is what normalization and vertical partitioning do, to differing extents). Each partition forms part of a shard, which may in turn be located on a separate database server or physical location.
source: Shard (database architecture)