Boney, WillCsima, Barbara F.Day, Nancy A.Harrison-Trainor, Matthew2020-07-072020-07-072019-03-21http://hdl.handle.net/10012/16042When classes of structures are not first-order definable, we might still try to find a nice description. There are two common ways for doing this. One is to expand the language, leading to notions of pseudo-elementary classes, and the other is to allow infinite conjuncts and disjuncts. In this paper we examine the intersection. Namely, we address the question: Which classes of structures are both pseudo-elementary and Lω1,ω-elementary? We find that these are exactly the classes that can be defined by an infinitary formula that has no infinitary disjunctions.enTechnical Report