The English word dozen comes from the old form douzaine, a French word meaning "a group of twelve" ("Assemblage de choses de même nature au nombre de douze" (translation: A group of twelve things of the same nature), as defined in the eighth edition of the Dictionnaire de l'Académie française).

Who invented the dozen?

The first to have used the unit were probably the Mesopotamians. 12 dozen (144 items) are a gross. 12 gross (1728 items) are called a great gross.

Why is a baker's dozen 13 instead of 12?

Baker's dozen means 13, instead of 12. The tale behind its origin is that a mediaeval law specified the weight of bread loaves, and any baker who supplied less to a customer was in for dire punishment. So bakers would include a thirteenth loaf with each dozen just to be safe.