Final answer:
The z-axis refers to the sagittal plane, which divides the body or an organ vertically into right and left sides.
Step-by-step explanation:
The z-axis refers to the sagittal plane. The sagittal plane is the plane that divides the body or an organ vertically into right and left sides. If this vertical plane runs directly down the middle of the body, it is called the midsagittal or median plane. If it divides the body into unequal right and left sides, it is called a parasagittal plane or less commonly a longitudinal section.