noun
Refers to a person who enters a specific building or facility, especially museums, libraries, and similar institutions. Often used in signs, announcements, and statistics.
入館者は受付で名前を記入してください。
Visitors, please write your name at the reception desk.
今月の入館者数は先月より増えた。
The number of visitors this month increased compared to last month.
訪問者 is a more general term for a visitor to a place or person, not limited to buildings.
Compound of 入館 (entering a building) + 者 (person). The exact historical derivation is straightforward from the kanji components.