Form taxon

A form taxon is a group of organisms based on what they look like (their morphology). Such a taxon may not reflect their biological relationships. In other words, they may not be a clade. 

A good example are the terms snake and lizard, which are not good taxonomic terms because neither is a proper clade. On the other hand, all poisonous snakes and lizards are in a good clade, the Toxicofera.Reeder, Tod W. et al 2015. Integrated analyses resolve conflicts over squamate reptile phylogeny and reveal unexpected placements for fossil taxa. PLoS ONE. 10 (3): e0118199. 

Another good example are ducks. They are in  the same family as swans and geese, but called "ducks" because of what they look like.

 References 

Category:Taxonomy