The full set: 东郊, 南郊, 西郊, 北郊 — one for each compass direction, all reading naturally as "[direction] suburbs." 郊区 is the generic "suburbs"; 北郊 specifies the northern side. Often used as a loose informal address before district boundaries existed.
history
In imperial times 北郊 also had a ceremonial sense — emperors performed the 北郊祭 (northern suburb sacrifice) to the Earth, paired with the 南郊祭 to Heaven. Beijing's 地坛 is the Ming-Qing altar for this rite.
