A printed circuit board that plugs directly into another plug-in card. For example, a mezzanine card, also known as a "daughterboard," might plug into a VMEbus card, CompactPCI card or PCI card in order to extend its functionality. Adapter cards are also available that enable a mezzanine card to plug into the larger slot of the target bus.

Smaller than standard expansion cards, mezzanine cards are typically designed for rugged industrial use and are often bolted down after being plugged in. IndustryPack, PMC, PC*MIP and PC/104+ are commonly used mezzanine cards. See IndustryPack, PMC, PC*MIP and PC/104.