In biology, protoplasm is the living substance inside the cell. At the simplest level, it is divisible into cytoplasm and nucleoplasm. It is also sometimes termed bioplasm, (Beale: meaning the essential substance of living matter within a cell) and is distinct from non-living cell components lumped under "ergastic substances". Ergastic substances can occur in the protoplasm. In many plant cells most of the volume of the cell is not occupied by protoplasm, but by "tonoplast": a large water filled vacuole enclosed by a membrane.

The idea that protoplasm is divisible into a ground substance called "cytoplasm" and a structural body called the Cell nucleus reflects most of the more primitive knowledge of cell structure that preceded the development of powerful micrographs of organic and inorganic substances, when it seemed that cytoplasm was mysteriously directed by the nucleus and controlled by the cell membrane. Today, it is known that the cytoplasm is structurally very complex, and that protoplasm is living because of the complexity of the "cytoplasmic organelles" and their careful separation and orchestration of multiple chemical processes.

Protoplasm exists in three forms: in the solid state, the liquid state and sometimes in a combined solid and liquid state.

Whether the protoplasm is in either of the three forms depends upon the physiological state of the cell.