A dozen (commonly abbreviated doz or dz) is a grouping of twelve.

The dozen may be one of the earliest primitive integer groupings, perhaps because there are approximately a dozen cycles of the Moon, or months, in a cycle of the Sun, or year. Twelve is convenient because it has a maximal number of divisors among the numbers up to its double, a property only true of 1, 2, 6, 12, 60, 360, and 2520.

The use of twelve as a base number, known as the duodecimal system (also as dozenal), originated in Mesopotamia (see also sexagesimal). Counting in base-12 can easily be accomplished on one's hands by counting each finger bone with one's thumb. Using this method, one hand can count to twelve, and two hands (with the second hand as a placeholder for representing units of twelve) can count to 144. Twelve dozen (12² = 144) are known as a gross; and twelve gross (12³ = 1,728, the duodecimal 1,000) are called a great gross, a term most often used when shipping or buying items in bulk. A great hundred, also known as a small gross, is 120 or ten dozen.

Dozen may also be used to express a moderately large quantity as in "several dozen" (e.g., dozens of people came to the party).

Varying by country, some products are packaged or sold by the dozen, often foodstuff (a dozen eggs).