An abbreviation for High Level Shader Language- A directX scripting language that puts most of the cool shit in your games, which most people turn off after their 8600 GT melts anyway.
Written almost like C and then structured like Assembly, there is loads of funky stuff you can do like create nightvision or post-process your frames in a buffer before putting them out to screen.
1: "Yo I was on nVidia
's site and they have like no support for HLSL, i wanted to put some depth of field in my new XNA game"
Developer 2: "Just like in Borderlands?"
Developer 1: "Yep"