A line-replaceable unit (LRU) is a black box of electronics, such as a radio or other auxiliary equipment for a complex engineered system like an airplane or ship. LRUs speed up repair, because they can be replaced quickly, restoring the big system to service. They also reduce the cost of systems, and increase the quality, by spreading development costs of the type of unit over different models of vehicles.
LRUs are designed to specifications. The specification defines the inputs and outputs. It also defines the tools to replace the unit (usually nothing more than a #2 Phillips-head screwdriver), and the bulk and weight (they always need to be carried by one man, and fit through a door, if possible). There are also requirements for flammability, unwanted radio emissions, resistance to damage from fungus, static electricity, heat, pressure, humidity, condensation drips, vibration, radiation and other environmental measurements.
Many LRUs for commercial aircraft are designed according to ARINC specifications. ARINC (Aeronautical Radio Incorporated) is a company owned by a number of airlines, that sells specifications and sets standards. LRUs are also defined by manufacturers like Airbus and Boeing, or various military organizations.
LRUs are painted crackle-black to more efficiently radiate and convect heat away from the unit.