- Overview of relevant material views.
- Evidence for existence of this material.
- Role of the observer.
It's been a journey, what materialists mean. See: Is the idea that "Everything is energy" even coherent?
I would relate the idea of the category 'matter' to the aim in scientific method to eliminate action-at-a-distance, in favour of particles colliding. Fields other than gravity can be related to potentials for interactions, like photons of the electromagnetic field to interact with moving charges. This programme has largely suceeded, although quantum nonlocality poses challenges.
Different people have meant different things, but matter in the physicalist-materialist scientific picture, I would relate to this programme. Crucially it dispels the 'mindliness' or personality and intentionality of the non-living world, which allows a great deal of ritual and superstition to be dispensed with, and a much sharper and more productive investigation to be made into the reliable influences we can have on the world.