my definitions probably aren't technically accurate, but I generally define it as Material + Texture(s) = Shader.

So, with that in mind, the entire connected nodes in the Hypershade (materials, textures, utilities, etc) that creates an effect I define as a Shader or Shading Group.

The material nodes (Lambert, Blinn, etc) are what I refer to as materials.

That's just me, though. But it works to keep it straight in my head when I explain it to others.