What terminology is used for the jelly-like substance within a cell?

Cytoplasm is the correct terminology for the jelly-like substance within a cell. It serves as the medium in which various cellular components, such as organelles, are suspended. The cytoplasm plays a crucial role in cellular processes, as it facilitates the movement of materials around the cell and supports the cell's structure. It contains various organelles, enzymes, and other substances critical for cell function and metabolism.

Regarding the other terms: the nucleus is a membrane-bound organelle that houses the cell's genetic material; the plasma membrane is the protective barrier that encloses the cell and regulates the movement of substances in and out of the cell; and cell fluid, while it may colloquially refer to cytoplasmic fluid, is not the specific scientific term used to describe the substance within the cell.