noun
during; throughout; all through
Used after a noun expressing a period of time to mean 'throughout that entire period' or 'all during that time'.
夏休みの間中、毎日海で泳いだ。
I swam in the sea every day throughout the summer vacation.
会議の間中、彼は一言も話さなかった。
He didn't say a single word during the entire meeting.
間 (あいだ) means 'between' or 'during' but does not inherently emphasize the entire duration; 間中 (あいだじゅう) explicitly means 'throughout the whole period'.
中 (ちゅう) as a suffix can mean 'during' or 'in the middle of', but 間中 (あいだじゅう) is more emphatic about covering the entire span.
Compound of 間 (あいだ, 'interval, period') and 中 (じゅう, 'throughout'). The reading じゅう is a rendaku form of ちゅう.