A BLT is a sandwich. The three letters are the three things inside it: bacon, lettuce, and tomato, usually with mayonnaise, often in toasted bread. It is café and diner English, informal, and very ordinary in American and British menus. People say "I'll have a BLT" the way they name any other sandwich; they almost never spell the foods out unless they are explaining it to someone new. Though actually, English uses BLT for other things too, but this word, in this entry, is only the sandwich. If the bacon is missing, it is not a BLT — the name is a short list of what you should find there.