A missile complex or missile silo was a structure in which missiles were housed ready for launch if needed. Such missiles included nuclear missiles or antimatter weapons.
Earth Missile Complexes[]
On Earth, missile complexes were part of the military infrastructure associated with World War III. After the war, Dr. Zefram Cochrane re-utilized one such site in Montana as a launch facility for his his Phoenix ship, itself a modified nuclear missile. (Star Trek: First Contact)
U.S.A.F. markings within the complex would seem to suggest that this facility was originally owned by the United States Air Force.
Otrin Missile Complexes[]
On Otrin's homeworld, missile silos housing defensive antimatter weapons were built after the Friendship 1 probe introduced antimatter technology to the locals. (VOY: "Friendship One")