The word junior

I'd like to know the meaning of the word 'junior' in the following sentence:

"He married with a girl who was twenty years his 'junior'.

Junior in this case is not his younger son, once he married in an advanced age, and never had had son.

It means the girl was twenty years younger than he.
If she is 20 years his junior and he is 50 years old then she is 30 years old.