The probability of homozygosity by descent (having common ancestors). The probability that a zygote obtains copies of the same ancestral gene from both its parents because they are related. The probability of autozygosity.
the probability that the two alleles present at a locus are identical by descent, i.e. are derived from the same ancestor.
the probability that two allelic genes united in a zygote are both descended from a gene found in an ancestor common to both parents; also the proportion of loci at which an individual is homozygous