What is the meaning of the word "front"?

Definitions:

a sphere of activity involving effort noun
- the Japanese were active last week on the diplomatic front
the atmospheric phenomenon created at the boundary between two different air masses noun
- A front passed through, bringing with it a sudden thunderstorm.
the outward appearance of a person noun
- They put up a bold front, but they had as many fears as the rest of us.
the part of something that is nearest to the normal viewer noun
- I walked to the front of the stage.