Packet injection
Packet injection is a computer networking term which refers to sending a packet on a network into an already established connection, usually by a party not otherwise participating in the said connection. This is accomplished by crafting a packet. Depending on the network medium and operating system, packet injection may be performed by using raw sockets, NDIS function calls, or by direct access to the network adapter kernel-mode driver. Sometimes IP address spoofing is used.
Software
A popular packet injection application for wireless networks is aireplay-ng[1], which is part of the aircrack-ng suite. Other packet-injectors are Nemesis, KisMAC, Ostinato, pcap, Winsock,T50, Void11, CommView for WiFi Packet Generator, file2air, AirJack, and libradiate.