A phylum of plants that has evolved to live in arid conditions.

If you're lost in a desert with nothing to drink, there can be up to a pint of water inside a cactus.
by robcraine November 23, 2005
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Slang term for being really out-of-it; caused by being overly tired, stoned, hungover, or just generally spaced-out, etc.
Person 1: "Wait, what time did you say he was coming over again? I keep forgetting..."

Person 2: "Wow, I've told you like three times.... Cactus!"

Person 1: "Haha, yeah- I'm mad cactus."
by Marti P. B. July 05, 2009
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A spikey plant found commonly in deserts. Cacti when refering to more than one.
You walk into the desert an there are fairly tall green spikey plants, these are cacti. A cactus on it's own in a desert is near impossible.
by [Hidden for privacy reasons] December 17, 2011
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