Fuse

A device designed to protect equipment and wiring.
Fuses work by creating the weakest link that will burn or break first, before the current becomes dangerous to the rest of the circuit.

A very good resource on common BUSS fuses
http://www.bussmann.com/library/arcflash/SPDFuseOperation.pdf

Fuses in homes and most applications have been replaced by Circuit Breakers. A fuse is likely more reliable in most RE applications than a Circuit Breaker.