Affinity (taxonomy) – mainly in life sciences or natural history – refers to resemblance suggesting a common descent, phylogenetic relationship, or type.
[1] The term does, however, have broader application, such as in geology (for example, in descriptive and theoretical works[2][3]), and similarly in astronomy (for example, see "Centaur object" in the context of 2060 Chiron's close affinity with icy comet nuclei.
Other resemblances we dismiss as being out of context or at least non-cogent; for example, in deciding whether to classify a lizard as having closer affinities to a snake than to a table, biologists rely on affinities such as the scales, blood, physiology, vertebral anatomy, and reproductive system as being more relevant than the possession of four "feet".
Analysing and determining the proper classification of an organism, a rock, or an astronomic object according to a particular system is often a difficult and treacherous procedure.
Problems in such fields of study have tripped up whole generations of workers in recent centuries.