black box
noun [ C ]
ukus [ usually singular ]
a system or process that uses information to produce a particular set of results, but that works in a way that is secret or difficult to understand:
The main problem with many international capital flows is that they are non-transparent - they come from, and reside within, a 'black box'.
a piece of electronic equipment used to record and store information:
Installation of a black box allows all emails sent and received to be monitored.
a piece of equipment on an aircraft that records the plane's movements and positions during a flight, and which can give important information if the plane has an accident:
The black box often holds critical clues for investigators looking for the cause of fatal plane crashes.