this institution; the main institution
Refers to the institution where the speaker/writer belongs (e.g., this hospital, this university) or the main/head institution of a group. Common in formal announcements and news.
This hospital has introduced the latest medical equipment.
The main hospital and the branch clinic have different consultation hours.
本院 is the main temple or headquarters of a temple organization, in contrast to 別院, its branch temple.
Compound of 本 (main, this) and 院 (institution). The reading ほんいん follows standard on'yomi.