noun, adverb
all summer; throughout the summer
Refers to the entire summer period. Often used adverbially with に or as a noun.
夏中、海で過ごした。
I spent the whole summer at the beach.
夏中にこの本を読み終えたい。
I want to finish reading this book during the summer.
夏休み refers specifically to summer vacation or break, while 夏中 means the entire summer season.
夏の間 also means 'during the summer', but is a phrase rather than a single word. 夏中 is more concise and emphasizes the whole duration.
Compound of 夏 (summer) and 中 (throughout, during). The reading なつじゅう follows standard rendaku for 中 when used as a suffix meaning 'throughout'.