Okay I will fly the Old Guy flag proudly and paste a definition:
A VTuber, or virtual YouTuber, is an online entertainer who uses a virtual avatar generated using computer graphics and real-time motion capture software or technology.
I'm in my forties and have known exactly what this was for years (ever since Hatsune Miku), but still had to look up the term "VTuber", which I did not recognize.
If you want to get nitpicky, Hatsune Miku is a voice synthesizer and character design that anyone can use, whereas VTubers usually have one person who "is" the character. A VTuber is basically the video equivalent of someone who uses a fake name on an internet forum; fans generally think of them as real people with coherent personalities. (Whereas any personality Hatsune Miku has is more like a loose convention among all the different artists who use the character.)
A VTuber, or virtual YouTuber, is an online entertainer who uses a virtual avatar generated using computer graphics and real-time motion capture software or technology.
Straight from Wikipedia.