ambassador to Japan
Refers to a foreign ambassador stationed in Japan. Often used in news and diplomatic contexts.
The new ambassador to Japan arrived at his post today.
The ambassador to Japan commented on the issue at the press conference.
大使 is a general term for 'ambassador', while 駐日大使 specifically means 'ambassador to Japan'.
駐日 is a prefix meaning 'stationed in Japan', used in compounds like 駐日大使 and 駐日米軍 (U.S. forces in Japan).
Compound of 駐 (ちゅう, 'to reside/stay'), 日 (にち, 'Japan'), and 大使 (たいし, 'ambassador').