Is Polygon a side chain or L2?

Can you clarify for me if Polygon is a sidechain or a Layer 2 solution? I've been hearing different opinions on this topic and I'm a bit confused. I understand that both sidechains and Layer 2s aim to enhance scalability and reduce transaction fees on blockchains, but I'm not entirely sure how Polygon fits into that categorization. Can you provide a concise explanation of Polygon's role and whether it's more accurately described as a sidechain or Layer 2?