Even in wikipedia, the general definition is there but obscured. Despite nearly every pictorial example in wikipedia featuring exposed concrete (as well as reddit's r/brutalist), any material can be found in a brutalist design. It must merely be exposed and accepted for what it is. e.g. wood shown to be wood and allowed to warp, decay, or even grow, as opposed to being overlaid with drywall. Stones not just in a foundation or a garden, but in a living room. This honesty and connection to materials is in contrast to the "dominating" sense of "brutalism," of which cheap government buildings are often twisted up in.
Though the pictures on wikipedia are misleading, certain quotes from the page are worth reading. Specifically when brutalism is described as "an ethic, not an aesthetic".
And also: "Peter Smithson believed that the core of brutalism was a reverence for materials, expressed honestly, stating "Brutalism is not concerned with the material as such but rather the quality of material",[33] and "the seeing of materials for what they were: the woodness of the wood; the sandiness of sand."
Source: wikipedia and have lots of architect friends.