black box

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A usually large, square room with black walls used as a space for theatrical performances.
• noun A device or theoretical construct with known or specified performance characteristics but unknown or unspecified constituents and means of operation.
• noun Something that is mysterious, especially as to function.

from the GNU version of the Collaborative International Dictionary of English.

• noun any electronic instrument or part of an instrument whose function is defined, but which is treated as a unit without consideration of the internal mechanisms; broadly, any device whose internal workings are considered as incomprehensible or mysterious by the user.
• noun (aviation) a device which maintains a record of cockpit conversations and the readings of many of the instruments on board an aircraft, continuously or at frequent time intervals during a flight. It is of rugged design to withstand a violent crash, and is used to determine the causes of aircraft accidents.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Informal name for the brightly colored cockpit voice recorders and flight data recorders in an aircraft designed to aid in determining the cause of an accident.
• noun A theoretical construct or device with known input and output characteristics but unknown method of operation.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun equipment that records information about the performance of an aircraft during flight