1. unit (e.g. meters, grams, etc.); denomination
それは、億よりも上の単位であった。It was a unit larger than a hundred million.
2. unit (i.e. one comprised of many); group; whole
クラスを一つの単位として行動する。We act with each class as a single unit.
3. credit (in school, university, etc.); point
ここで単位を取らなければ卒業できないのだから。Because if you don't get the credits here, you won't be able to graduate.
4. by the ...; in units of ...; in groups of ...; in amounts of ...; each ...; by ...; every...
as ~で