chargé d'affaires

1. A diplomat who temporarily substitutes for an absent ambassador or minister.
2. A diplomat of the lowest rank, accredited by one government to the minister of foreign affairs of another.

[French : chargé, charged, in charge + de, with, of + affaires, affairs.]

The noun has one meaning:

Meaning #1: the official temporarily in charge of a diplomatic mission in the absence of the ambassador