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Definition: multicast

(1) To transmit data to multiple recipients on the network at the same time using one transmission stream to the switches, at which point data are distributed out to the end users on separate lines.

Contrast with Unicast
A "unicast" is a separate transmission stream from source to destination for each recipient. When sending large volumes of data, multicast saves considerable bandwidth over unicast.

Contrast with Broadcast
A "broadcast" is a transmission to every node reachable on the network or subnet, rather than to a specific list of nodes as in a multicast. See IP multicast, IGMP and SRM.

(2) In digital television broadcasting, to send multiple standard TV (SDTV) programs in the allotted bandwidth, rather than one high-definition TV (HDTV) program.