Wikipedia
A SYN flood is a form of denial-of-service attack in which an attacker rapidly initiates a connection to a server without finalizing the connection The server has to spend resources waiting for half-opened connections, which can consume enough resources to make the system unresponsive to legitimate trafficThe packet that the attacker sends is the SYN packet, a part of TCP's three-way handshake used to establish a connection
SYN_flood - Wikipedia, the free encyclopedia