What is a mantel?

Excuse me, could you possibly clarify something for me? I've come across the term 'mantel' quite frequently in recent discussions about interior design and home furnishing, but I'm afraid I'm not entirely certain what it refers to. Could you explain, in a nutshell, what a mantel actually is? Is it a specific type of furniture? A decorative element? Or perhaps a more architectural feature of a fireplace or some other part of a room? I'd really appreciate it if you could shed some light on this matter for me.