tl;dr: PIOPs, as originally introduced in the DARK paper1. (Although I think they were kind of already defined in PLONK2 and Marlin3?)
References
For cited works, see below 👇👇
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Transparent SNARKs from DARK Compilers, by Bünz, Benedikt and Fisch, Ben and Szepieniec, Alan, in Advances in Cryptology – EUROCRYPT 2020, 2020 ↩
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PLONK: Permutations over Lagrange-bases for Oecumenical Noninteractive arguments of Knowledge, by Ariel Gabizon and Zachary J. Williamson and Oana Ciobotaru, in Cryptology ePrint Archive, Report 2019/953, 2019, [URL]Â ↩
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Marlin: Preprocessing zkSNARKs with Universal and Updatable SRS, by Alessandro Chiesa and Yuncong Hu and Mary Maller and Pratyush Mishra and Noah Vesely and Nicholas Ward, in Cryptology ePrint Archive, Report 2019/1047, 2019, [URL]Â ↩