1. The term "broadcast" is generic and is used in many different area. The origin of the term obviously means to cast out broadly, such as a radio broadcast. Subdefinition: Ethernet has broadcast domains, allowing you to partially sniff some data from your neighbors, and possibly subvert it. Typical protocols that can be sniffed and subverted in this manner are: ARP, NetBIOS, MSBROWSE, rwho, bootp/DHCP, SNMP. An Ethernet broadcast address is "FF:FF:FF:FF:FF:FF". Subdefinition: The Internet protocols TCP/IP support a feature known as a directed broadcast, which allows a remote person the ability to send a single packet to an entire subnet. This will then take advantage of the Ethernet broadcast domain once it reaches its destination. Attacks like smurf take advantage of this. A directed broadcast address looks something like 192.0.2.255, where the last integer "255" means "all devices on subnet 192.0.2.x". Subdefinition: The special IP address of "255.255.255.255" is the local broadcast, and causes the packets to be sent to everyone locally, but not across the Internet. From Hacking-Lexicon

2. a type of communication between hosts (or computers) on a network where a computer can talk to all computers. See multicast and unicast. From Linux Guide @FirstLinux