noun. Cytoplasm is the substance that fills living cells. Cytoplasm is the substance that fills living cells. The definition of cytoplasm is the clear, gel-like substance outside the nucleus of the cell of plants and animals.An example of cytoplasm is the substance that fills every living cell in our bodies.ytoplasm is the substance that fills living cells. The definition of cytoplasm is the clear, gel-like substance outside the nucleus of the cell of plants and animals.