People often use the terms RTL and HDL interchangably.
I see it in a bit of a different way, influenced by Carver & Mead’s book “Introduction to VLSI systems”. It’s more of a design methodology.
For reasons of speed and debugging, it makes sense to split up the data processing into smaller steps and buffer the results in registers (small memories usually made up of a stack of flip-flops). As the data flows between registers, it’s called register transfer level.
Here’s a quote from page 105 of the pdf of the book:
For a different viewpoint, this thread in stackexchange tries to answer the question - what is the difference between RTL and HDL
I would recommend this course to people who don't have any awareness of open source hardware development. It can be a bit of an eye opener, a lot of people in industry have a disconnected impression of what the landscape really is.