I can offer a definition from my personal experience. In many cases synonymous with the "San Francisco Burrito" or "Mission Burrito", this is a large portable meal(s) consisting of meat, cheese, rice, beans, salsa (hot sauce, pico de gallo, or both), sour cream and certain optional ingredients (frequently lettuce, minced cilantro, or guacamole), wrapped in a large (usually 14-inch) tortilla, further wrapped in two layers of aluminum foil. It typically weighs 0.5 - 1 kg, contains >1500 calories, and by many is—indeed can only be—eaten in intervals over the course of a day. It is most often purchased from a taqueria or taco truck. Several Mexican-American chain restaurants, notably Chipotle, have adopted versions into their menus.