The term "isoplanatic" refers to an imaging system or process where the imaging properties do not change over a certain distance in the image plane, i.e. are "shift invariant". There might be image imperfections (aberrations), but they are identical over that distance
The term "isoplanatic" is derived from Ancient Greek πλανάω (planáō, “wander about, stray”),see "planet". Isoplanatic can then be interpreted as: having equal/identical aberrations. Noun is: isoplanatism
Source for the etymology: "An Etymological Dictionary of Astronomy and Astrophysics English-French-Persian". The dictionary is published in the internet. Source for the definition:
Max Born, Emil Wolf, Principles of Optics, Cambridge University Press, 7.ed., 1999, p.545