Many old definitions of material and immaterial leave something to be desired.
For example, how does a (supposedly) sizeless electron fit into Descartes' world of res extensae and res cogitantes?
A more modern approach is to use the term physical. A photon is sizeless and even massless (no rest mass), but it is physical, because it can be observed. The debate in modern philosophy is then if "stuff" like qualia may be non-physical.
Now, from my understanding of Aristotle, he didn't have a concept of qualia and regarded the whole "sensing something" as a completely material process.
Yet he argued that the intellect is immaterial...
So, what is it that makes matter matter for Aristotle? What is his definition of material and immaterial?
It seems that matter for Aristotle encompasses everything physical (in the modern understanding) and even more.