In geology, the fine-grained or glassy part of a porphyritic rock, as opposed to the large crystalline phenocrysts.
Also, the coarse and fine mineral material that forms the basis of soils. It is the main inorganic component of the soil, remnant of the original weathered bedrock, as opposed to pedofeatures, which are distinct minerals in smaller concentrations in the soil. Except groundmass sometimes refers to the base minerals of pedofeatures as well as the base minerals of rocks. Also, sometimes even if the groundmass is impregnated with foreign minerals it's still called groundmass.
The very fine particles in soil groundmass are called the micromass.
Everything's so much clearer now.