I believe it is of Latin root, but we know it means "equal", or "equal to". Weather forecasters commonly show "isobars" which represent lines or colors of equal value pressure. Same concept in simulation. Isosurfaces will connect all points of equal value with one surface. You get to pick the value. Hope this helps. - Tony
I like Anthony's answer. Are you referring to Iso Clipping in Simultion or Iso Surface in Flow?
The Iso Clipping will simply act as a high- or low-pass filter, showing you only the volume in your model that has a response higher or lower, respectively, than the value in iso clip definition. This is a great way in larger assemblies to show critical areas of the model clearly (e.g. only show stress > yield strength or FOS < 1, 2, 3, etc)