throughout the afternoon; all afternoon
Refers to the entire duration of the afternoon, often implying continuity or persistence.
He was in meetings all afternoon.
午後中雨が降り続いた。
It kept raining throughout the afternoon.
一日中 means 'all day long', covering both morning and afternoon, whereas 午後中 is limited to the afternoon.
Formed from 午後 “afternoon” and the suffix 中 “throughout.” In this temporal use, 中 is read じゅう; this is a distinct suffix reading, not sequential voicing of ちゅう.