下面是它們的英文定義：
A northbound interface is an interface that conceptualizes lower level details. It interfaces to higher level layers and is normally drawn at the top of an architectural overview.
A southbound interface decomposes concepts in the technical details, mostly specific to a single component of the architecture. Southbound interfaces are drawn at the bottom of an architectural overview.
Northbound interfaces normally talk to southbound interfaces of higher level components and vice versa.
從英文定義可以看出來：
1.北向接口是某個模塊的頂層抽象接口；
2.南向接口是某個模塊之內部子模塊的接口；
3.北向接口因處於架構圖的頂部而得名，南向接口則因處於架構圖的底部而得名，所謂上北下南；
4.另外，北向和南向也是相對的，相對於更頂級的模塊，某個模塊的北向接口就相對來說是南向接口了。