The California DOT says they are synonymous[1]: >High-Occupancy Vehicle (HOV) lanes, also known as carpool or diamond lanes [1] - https://dot.ca.gov/programs/

The California DOT says they are synonymous[1]: >High-Occupancy Vehicle (HOV) lanes, also known as carpool or diamond lanes [1] - https://dot.ca.gov/programs/traffic-operations/hov#:~:text=What%20Is%20an%20HOV%20Lane,carrying%20capacity%20of%20California%20highways https://dot.ca.gov/programs/traffic-operations/hov#:~:text=W....