relative who is two meioses away from a particular individual in a pedigree;
a relative with whom one quarter of an individual's genes is shared (i.e.,
grandparent, grandchild, uncle, aunt, nephew, niece, half-sibling)
Definition from: GeneReviews
from the University of Washington and the National Center for Biotechnology Information
Related discussion in the Handbook
See also Understanding Medical Terminology.