Final answer:
The endoplasmic reticulum is not used to define the structure that separates the contents of a human cell from its surrounding medium; it is involved in protein and lipid synthesis within the cell.
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 (d) Endoplasmic reticulum. The plasma membrane, also known as the cell membrane, is the lipid bilayer that encloses the cell, acting as a selective barrier allowing some molecules to pass while restricting others. The cell wall is usually found in plant cells and certain prokaryotes, providing structural support and protection, and is not a feature of human cells. The nuclear envelope is a membrane that surrounds the nucleus, separating it from the cytoplasm. However, the endoplasmic reticulum, while indeed being a membrane system within the cell, it does not separate the cell's contents from the surrounding medium, but rather functions as a site for protein and lipid synthesis.