Final answer:
The term 'cytoplasm' does not define the structure that separates the contents of a human cell from its surrounding medium; rather, it is the internal environment of the cell that contains organelles such as ribosomes.
Step-by-step explanation:
The term that is not used to define the structure that separates the contents of a human cell from its surrounding medium is cytoplasm. The plasma membrane, also known as the cell membrane or plasmalemma, is the structure that forms the physical boundary between the cell and its environment. The cell wall is a feature often found in plant cells and some prokaryotic cells that provide rigidity and protection but is not present in typical human cells. The nucleus is an organelle that contains the genetic material and is enclosed by its membrane; however, it does not separate the entire cell from its surroundings. Therefore, the cytoplasm, which refers to all of the cellular material inside the plasma membrane and includes structures like the ribosomes, is the correct answer as it is not a structure that separates the cell from its surroundings.