What kind of port scan is this if an ethical hacker sends TCP packets to a system that has the SYN flag set, on open ports, none of the SYN/ACK replies are being responded to?

ECC describes a stealth scan as what most of us used to call a half-open scan (although I think it would make more mathematical sense to call it a two-third scan, given it's a three-way handshake and only two are utilized). This is sometimes referred to as a SYN scan.