The labrum is a flap-like structure that lies immediately in front of the mouth in almost all extant Euarthropoda

The most conspicuous exceptions are the Pycnogonida, which are probably chelicerates. In entomology, the labrum amounts to the "upper lip" of an insect mouth, the corresponding "lower lip" being the labium.

The evolutionary origin, embryogenesis, and morphological development of the labrum have proved to be some of the most controversial and challenging topics in the study of arthropod head structures.