Daughterboard, shield, hat, cape, ...
==

The standard name for a subsidiary printed circuit board (PCB) that plugs into a main PCB is "daughterboard"

However, the somewhat artsy communities that produce these small experimental microcontroller boards each tend to invent their own name for daughterboards that have the required dimensions and connector to mate with the primary form of main PCB in that community

    Arduino:       shield.
    Raspberry pi:  hat.
    Beaglebone:    cape.