Often used to describe bands like Pantera and Exhorder. With mid-tempo riffs similar to those found in Thrash Metal and a tendency to focus more on groovy syncopation and heaviness than outright speed.
I love that album. It's a natural step forward from Symbolic. The songwriting is a lot more open and sprawling, so it can take some getting used to. My favorite of Chuck's vocal work. It's also the most modern-sounding, something to bear in mind when you listen to their older stuff. Individual Thought Patterns has the weakest mix of them IMO, but the music rules.
As far as the roots of DM, Death was one of the originators, but "Seven Churches" by Possessed (1985) usually gets the nod as the first death metal studio album past the gate.