下面是它们的英文定义：
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.
从英文定义可以看出来：
1.北向接口是某个模块的顶层抽象接口；
2.南向接口是某个模块之内部子模块的接口；
3.北向接口因处于架构图的顶部而得名，南向接口则因处于架构图的底部而得名，所谓上北下南；
4.另外，北向和南向也是相对的，相对于更顶级的模块，某个模块的北向接口就相对来说是南向接口了。