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1.
The feeling you get when you have to go to the bathroom.
"Nature's callin" or "nature calls" is usually how nature's call is said in a sentence.

Guy #1: Dude, where you going so fast?

Guy #2: Nature calls!
by chocolate box July 22, 2006
 
2.
the need to use the toilet
I awoke to nature's call in the middle of the night.
by Light Joker January 18, 2007