What is a soft 404 error in SEO?

@terrence 

A soft 404 error in SEO occurs when a webpage is returned with a "200 OK" status code, even though it should ideally return a "404 Not Found" status code. This typically happens when a webpage is no longer available, but the server still returns a valid page instead of the proper error message. Soft 404 errors can negatively impact SEO as search engines may continue to index non-existent pages, leading to poor user experience and potential loss in rankings.