RFC 919, "Broadcasting Internet Datagrams," Section 6 (Gateways and Broadcasts), defining the distinction between local and directed broadcasts.
RFC 1112, "Host Extensions for IP Multicasting," Section 1, defining multicast as the delivery of datagrams to a "host group" (zero or more hosts).
Cisco Systems, "IP Addressing: IPv4 Addressing Configuration Guide," Configuring IP Directed Broadcasts, which defines directed broadcasts as packets sent to the broadcast address of a subnet to which the sender is not directly attached.
RFC 1812, "Requirements for IP Version 4 Routers," Section 4.2.2.11, discussing the requirements for forwarding directed broadcasts versus limited broadcasts.