This paper attempts to define a vihāra “monastery” in the premodern Cambodian context on the basis of existing epigraphical data and archaeological findings. After analyzing about twenty inscriptions which mention the term vihāra (dated 7th–14th centuries CE and composed in Old Khmer, Sanskrit and Pali), it discusses the physical aspects of the vihāras, with references to two sites of monasteries which have relatively rich archaeological remains; viz., the Royal Monastery (present-day Ta Prohm temple, Siem Reap province) and the vihāra at Prey Svay (Sukhothai province, Thailand). The paper also examines the operational aspects of the vihāras by focusing on their three functions: place of dwelling, place of learning and place of worship.