Is Optimism arbitrum?

Could you please clarify for me if Optimism is synonymous with Arbitrum? I'm trying to understand the difference between these two terms in the context of cryptocurrency and blockchain technology. I've heard they're both related to scalability solutions, but I'm not quite sure if they're interchangeable or serve different purposes. Could you elaborate on the distinctions between Optimism and Arbitrum, and explain their respective roles in the crypto ecosystem? Thank you for your assistance in clarifying this matter.