What do biologists mean when they use the term "biological distance"?
Biological distance is pretty much what you'd guess: how closely related are x and y based on genetic (phenotype or genotype) studies?

There is zero biological distance between identical twins. There is extremely little biological distance between family members (for most intents and purposes, none, no matter where they live). It goes on and on in this manner.


  Biological distance, or degree of genetic relatedness, will be assessed by a comparison of dental nonmetric trait frequencies.1  


The phrase has been used in Theoretical Population Biology for decades. Back then, they had phenotypes to measure the degree of biological distance.2 Today, actual genetic determination of kinship is possible.


  Biological distance, or biodistance, is the measure of biological relatedness (or divergence) between and within human groups, living and past, based on human skeletal and dental variation.3


1 Dental Variation of Prehistoric Amerindian Populations
from Eastern Tennessee
2 Kinship, information and biological distance
3 Biological Distance in Bioarchaeology and Human Osteology