What is called an attack in which an attacker floods a system with connection requests...

A SYN attack occurs when an attacker floods the target system's small "in-process" queue with connection requests, but it does not respond when the target system replies to those requests. This causes the target system to "time out" while waiting for the proper response, which makes the system crash or become unusable. A buffer overflow attack occurs when a process receives much more data than expected. One common buffer overflow attack is the ping of death, where an attacker sends IP packets that exceed the maximum legal length (65535 octets). A smurf attack is an attack where the attacker spoofs the source IP address in an ICMP ECHO broadcast packet so it seems to have originated at the victim's system, in order to flood it with REPLY packets.

Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, John Wiley & Sons, 2001, Chapter 3: Telecommunications and Network Security (page 76).