Cloud's Transitive Principle of Y-DNA Kinship
If Y-DNA results indicate two people are not related (i.e. genetic distance calculations are unfavorable) yet each person is related to a third person, then they are also related to each other.
This applies to the results of "mono-lineal" DNA tests that trace one line only – either mtDNA or Y-DNA. Autosomal and X-chromosome DNA tests won't work as they return data on all lines and two people might be related but the third person might be related on a different line.
If the Y-DNA indicates that Alvin is related to Bruce, and that Bruce is related to Carl, then Alvin is related to Carl somewhere along their paternal (patrilineal) line. The same principle applies to mtDNA testing, except the relationship exists on the maternal (matrilineal) line.
This definition is modeled after the mathematical Transitive Principle of Equality:
If A = B
and B = C
then A = C
This only applies to Y-DNA and mtDNA predicted relationships.
Definition by Tom Cloud as a joke, but it is a practical expression of the application of DNA data in genealogical research – if the DNA is inconclusive about a relationship, perhaps an intermediate relation can be found to confirm it.