They are literally just known as "non-benders".
From Tenzin's opening monologue in the first episode of Legend of Korra:
Avatar Aang and Fire Lord Zuko transformed the Fire Nation colonies into the United Republic of Nations, a society where benders and non-benders from all over the world could live together, in peace and harmony.
And a quote from Pema in that same episode, illustrating that non-benders refer to themselves by that term as well:
All I want is one child like me, a nice non-bender, who doesn't blast wind in my face every five seconds.
As found by Valorum, the show's creators Michael Dante Dimartino and Bryan Konietzko also referred to them as "non-benders" in an interview.
(source for the quotes - obligatory TV Tropes warning)