junction box

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

• noun An enclosure within which electric circuits are connected.

from The Century Dictionary.

• noun In electricity, the appliance used to connect underground or concealed electric conductors.
• noun A chamber connecting two or more lines of pipe.

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

• (Elec.) A box through which the main conductors of a system of electric distribution pass, and where connection is made with branch circuits.