A unit of data sent across a network. When a large block of data is to be sent over a network, it is broken up into several packets, sent, and the reassembled at the other end. Packets often include checksum codes to detect transmission errors. The exact layout of an individual packet is determined by the protocol and network architecture being used. In many cases, it could be also called a sub-unit of a data stream; a grouping of information that includes a header (containing information like address destination) and, in most cases, user data.