The singular form of cactus is simply "cactus." Yep, you read that right! It's a bit of a funny word because, unlike many English words, it doesn't change when we're only talking about one.
Now, you might be thinking, "Wait, isn't that wrong? Shouldn't it be 'cacti' or something?" And you're not entirely wrong - when we're talking about multiple cacti, we usually add an "-i" to the end to indicate plurality (as in, "I saw a bunch of cacti on my trip to the desert").
But when we're talking about just one spiky plant, it's perfectly correct to use "cactus" all by itself. It might seem a little weird at first, but it's just one of those quirks of the English language that we get used to over time.
Hope that helps!
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