Is arbitrum one the same as arbitrum?

Could you please clarify for me if "arbitrum one" and "arbitrum" refer to the same entity or concept? I'm trying to understand the nuances in the cryptocurrency and finance world, and this particular detail seems to be causing some confusion. Is it possible that these two terms are interchangeable, or do they have distinct meanings and applications? Your expertise in this area would be greatly appreciated in helping me to clarify this matter.