Parcentric is used to describe microscopes. It is the center of the field that does not go outside the inner one third of the field of view with any objective of a matched set.
Microscope lenses that are parfocal and parcentric allows the user to changed between lenses will keeping the speicemen focused and centered, instead of having to adjust the x, y or z plane (i.e. focus and the stage).
Look up parafocal and parcentric.