From belt +‎ -er.

Belter (plural Belters)

1. (science fiction) An inhabitant of an asteroid belt, especially that between Mars and Jupiter.
   • 1966 February, Larry Niven, “The Warriors”, in If, volume 16, number 2 (#99 overall), page 152:

     You noticed a habit of mine once. I never make gestures. All Belters have that trait. It's because on a small mining ship you could hit something waving your arms around.

   • 2015 December 22, Remembering the Cant (The Expanse), season 1, episode 3, spoken by Jim Holden (Steven Strait):

     Earth and Mars have been stepping on the necks of the Belters out here for over 100 years and I didn't want to be the boot.

   • 2019 December 22, Annalee Newitz and Cyrus Farivar, “Nick Farmer knows dozens of languages, so he invented one for The Expanse”, in Ars Technica‎^[1]:

     He also decided that Belters wouldn’t use the term “bullshit,” because there would be no bulls in the Belt. Instead, they would come up with a uniquely Belter curse, “kaka felota,” a term for what happens when your toilet backs up in low gravity and shit floats all over the place. (“Kata felota” literally means “floating shit.”)

• Solarian

Belter

1. Ellipsis of Belter Creole.
   • 2019 December 22, Annalee Newitz and Cyrus Farivar, “Nick Farmer knows dozens of languages, so he invented one for The Expanse”, in Ars Technica‎^[2]:

     To communicate, they evolve a creole called Belter, which becomes the lingua franca for what is essentially the solar system’s new proletariat. […] The resulting Belter creole is a crazy mix of English, Chinese, romance languages like French, German, Persian, Hebrew, Zulu, and a few other surprises. Farmer says he has over 1,000 Belter words in his personal dictionary, and he keeps adding more as the show’s producers and fans request them.

• Jesse Sheidlower, editor (2001–2026), “Belter n.”, in Historical Dictionary of Science Fiction.

• Beltre, Elbert, Lebert, treble