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.
Parcentric refers to the alignment of optical elements in a microscope so that the center of the field of view remains constant when changing objectives. Parfocal means that when changing objectives on a microscope, the image stays approximately in focus.
If a microscope is parcentric, it means that as you switch between different objective lenses, the specimen will remain centered in the field of view without having to readjust its position. This ensures that the specimen stays in focus and properly aligned as you change magnifications.
Look up parafocal and parcentric.
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).