[Crustacea] The phase of crustacean larval development characterised by three pairs of functional appendages: uniramous first antennae, biramous second antennae and mandibles. The body is oval and unsegmented. There are two stages in this phase. The nauplius is the larval form which hatches from the eggs of species which shed their eggs freely into the water.

Alternative forms for nauplius : naupliar, nauplii.