Um, I'm pretty sure the information you've been given so far is incorrect.
Split Squat and Lunge are exactly the same thing, just different words used to describe the same movement.
The above posters have referenced different types of split-squats/lunges:
If your hind foot is up on a box, then it's a Bulgarian split-squat/lunge.
The other differences mentioned above are between a dynamic and static split-squat/lunge.
From what I recall, the movement was originally called split-squat and considered a variation of the squat, but it gained popularity over time and eventually earned its own name.