Nurbs, or non-uniform rational b-splines, to be specific
It's a generalization of B-splines, called U-splines (splines over Unstructured meshes). If anyone is interested in the math behind it on the level of a journal article, the last two links here go into the technology in detail: https://docs.coreform.com/latest/html/coreform-public/index....
(Disclaimer: I work for Coreform, developing U-splines)
Going by the name 'isogeometric analysis' is is probably analysis on level-sets, not sure if splines are the easiest to translate to level sets but the effect is probably very similar.