Primary teeth: The first teeth which are shed and replaced by permanent teeth. The first primary tooth comes in at about 6 months of age and the 20th and last primary tooth erupts at around 2 1/2 years of age. The primary teeth are replaced beginning usually at about age 6. Also called baby teeth, milk teeth, temporary teeth or deciduous teeth. (In Latin, decidere means to fall off or be shed, like leaves from a tree).