A Mexican dish made by filling a tortilla (corn or flour) with cheese, folding in half and toasting until the cheese is melted. Additional ingredients, such as meats (chicken or beef, commonly), or vegetables can be added. Often served with salsa, guacamole and/or sour cream.
Definition #2 for "quesadilla"
A quesadilla is a flour tortilla or a corn tortilla filled with a savory mixture containing cheese, other ingredients, and/or vegetables, then folded in half to form a half-moon shape. This dish originated in Mexico, and the name is derived from tortilla and the Spanish word for cheese queso.