Oxford Reference is a massive multidisciplinary encyclopedic database that includes 25 specialized encyclopedias in the disciplines of art and architecture. Among them is the Oxford Companion to Architecture, which has entries written by architects and scholars on topics in their fields, provides bibliographic information for further reading, and links to related articles.
SAH Archipedia is an authoritative encyclopedia of the built world published by the Society of Architectural Historians and the University of Virginia Press. It contains histories, photographs, and maps for more than 14,000 structures and places. Though primarily a building database, you will also find landscapes, infrastructure, monuments, artwork, and more.