It used to be "M-DISC", with the 'M' meant to mean "millenium": as in, they'd last 1000 years.

I somehow thought the company making the M-DISCs was dead but apparently Verbatim (and others) still sells BluRay discs labelled as "M-DISC" and they're compatible with many BluRay readers/writers.

You can still buy M-DISC branded discs - either blu-ray or DVD. I think it matters more for DVD than for blu-ray because blu-ray discs use a more reliable recording medium anyway. People say there's little if any difference between M-DISC BDXL and 'normal' BDXL. You can get 100GB or 128GB BDXL discs.

There are two different Blu-ray dye technologies, LTH and HTL. HTL is the original and uses more expensive inorganic materials (germanium, bismuth, or palladium) and then there's the cheaper LTH that came later and uses an organic dye that's more susceptible to light, breaks down rather quickly over time, and has more burning/reading issues. You don't want to use the latter.

Typically if the surface of the disc is gold and darkens with writing, it's an LTH disc.