Paracentric chromosome inversion: A basic type of chromosome rearrangement. A segment that does not include the centromere (and so is paracentric) has been snipped out of a chromosome, turned through 180 degrees (inverted), and inserted right back into its original location in chromosome. The feature that makes it paracentric is that both breaks are on the same side of the centromere so that the centromere (the point at which the chromosome attaches to the spindle) is not affected.
An inversion can be "balanced," meaning that it has all the genes present in the normal uninverted chromosome. Or an inversion can be "unbalanced," meaning that genes been have deleted (lost) or duplicated.
A balanced inversion in a child causes no problems. An unbalanced inversion is abnormal and is often associated with problems such as development delay (and later, mental retardation) and multiple congenital anomalies (birth defects).