In connective tissue, the ground substance is an amorphous gel-like substance surrounding the cells. In a tissue, cells are surrounded and supported by an extracellular matrix. Ground substance traditionally does not include fibers (collagen and elastic fibers), but does include all the other components of the extracellular matrix.
The components of the ground substance vary depending on the tissue. Ground substance is primarily composed of water, glycosaminoglycans (most notably hyaluronan), proteoglycans, and glycoproteins. Usually it is not visible on slides, because it is lost during staining in the preparation process.
The meaning of the term has evolved over time.
In cytology, it may refer to the cytosol or protoplasm.
References
Ground substance Wikipedia(Text) CC BY-SA