TCP SYN flood

Definition
A TCP SYN flood is

"a type of DDoS attack, conducted by taking advantage of the flaw of TCP three-way handshaking behaviour. The attacker makes connection requests aimed at the victim server with packets with unreachable source addresses. The server is not able to complete the connection requests and, as a result, the victim wastes all of its network resources. A relatively small flood of false packets will tie up memory, CPU, and applications, resulting in shutting down a server."