If you consider a knotless tapered leader, it is the bit of mono on the end which can be as long as 2 feet. This is generally known as a tippet.
The idea of the tippet is maily confined to river fishing and it is an American term. The old British term, a cast, was the length of gut, latterly mono, fastened to the end of the fly line.
During the 60s, this term was changed to a leader. I do agree it's a bit confusing for a beginner.
I don't use knotless tapered leaders for still water fishing anymore. For river fishing I tend to use a tapered braided leader with a finer "tippet" tied on to the end, especially using dry flies.
Hope this helps.