A type of symbiotic relationship in which one partner benefits and the other is unaffected. As opposed to parasitism where the other is harmed.

There are many commensal relationships in biology. Many birds benefit by living in trees, but have little affect on the tree.

Another example of commensalism is the flatworm and the horseshoe crab. The flatworm attaches itself to the gills of the crab and benefits by eating scraps of food that elude the crab during its feeding, but the crab is unaffected.