Bjork, Robert., ed. The Oxford Dictionary of the MIddle Ages. 4 vols. Oxford: Oxford University Press, 2010.
Online only (click on title, maximum of 5 simultaneous users). Covers "all key aspects of European history, society, and culture from 500 to 1500 A.D., as well as the Byzantine Empire, Islamic dynasties, and Asiatic peoples of the era." Designed for medievalists, students, and general readers.
Kazhdan, Alexander P., ed. The Oxford Dictionary of Byzantium. 3 vols. New York; Oxford: Oxford University Press, 1991.
Ref. DF521.O93 1991
Available both in print and online (click on title above, maximum of 5 simultaneous users). According to the preface, this is the "first attempt to collect within a single work data concerning all fields of Byzantine studies." Perhaps because of this attempt at comprehensiveness, religious history is deemphasized. Though useful, usually possible to obtain fuller coverage in either the Lexikon des Mittelalters (see next entry) or Dictionary of the Middle Ages. See Johnston, Reference Books, p. 171.
Lexikon des Mittelalters. 10 vols. Munich and Zurich: Artemis, 1977- 1999.
The most comprehensive encyclopedia of the middle ages in any language. Has a more scholarly thrust than the Dictionary of the Middle Ages (see below). Excellent bibliographies. See Johnston, Reference Books, p. 185f.
Melville, Gert and Martial Staub, eds. Brill's Encyclopedia of the Middle Ages. 2 vols. Leiden; Boston: Brill, 2017. Ref. CB351 .E5913 2017
English translation of the second edition (2013) of the Enzyklopädie des Mittelalters, published by Primus Verlag. The encyclopedia is dedicated to all aspects of medieval life
Strayer, Joseph R., ed. Dictionary of the Middle Ages. 13 vols. New York: Scribner's, 1982-1989.
See the discussion in Johnston, Reference Books, p. 186-187. The most comprehensive English-language encyclopedia for the medieval period. Although (or perhaps, because) it covers a well-nigh comprehensive range of medieval topics, it does better at offering introductory surveys than penetrating research. Nevertheless, a good starting point, especially to become familiar with medieval topics new to you. Has an excellent index volume.
Vauchez, Andre, ed. Encyclopedia of the Middle Ages. 2 vols. Chicago and London: Fitzroy Dearborn Publishers, 2000.
Translation of Dictionnaire Encyclopedique du Moyen Age (in Pius at CB351.D5 1997); compared to the Dictionary of the Middle Ages and Lexikon des Mittelalters coverage is restricted to "medieval Christendom" and "peoples and religions that were in contact with it" including Jews and Muslims. Written primarily for a European audience and intended to help such an audience "identify with an inheritance that still marks their way of life and some of whose aspects still charms them, but whose meaning escapes them," the encyclopedia gives a privileged place to philosophy, theology, spirituality, liturgy and iconography."