In reality, a triangle square is the result of joining two half square triangles (hst's) and is defined as such in all of my older basic quilting books. It's a much more accurate term for what most people now refer to as HST's.
It's not a new term at all and is frequently used in class instruction and personal conversations. If most of your quilting 'connections' are visual instead of verbal, it's understandable that you may not have run across it before this. I suspect 'HST' became an acceptable, though inaccurate, substitute as a result of message boards and blogs simply because it's much faster to type than 'triangle squares'.