Is kelp a plant or algae?

Excuse me, I've been pondering over a question for quite some time. Could you please clarify for me whether kelp is categorized as a plant or an algae? I've heard conflicting views on this, and I'm quite curious to know the accurate answer. Could you possibly explain the basis for your classification, so I can have a better understanding of this matter? It would be greatly appreciated if you could shed some light on this matter for me.