Egg toast is an open sandwich: a slice of toast with an egg preparation on top, eaten with the egg facing up rather than mashed into salad and hidden between two slices. That openness is the whole identity. Because nothing covers the egg, the egg is the dish and the toast is its plate, and the two have to be matched so the bread can carry a soft, often runny topping without going limp or being overwhelmed. A fried egg, a folded omelet, scrambled curds, or a Japanese rolled tamagoyaki can all sit on it, but in every case the defining tension is the same: a sturdy, well-toasted base under a tender egg, each chosen for the other.
The craft is in the toast as much as the egg. The bread is usually thick-cut shokupan, toasted until the surface is crisp and the inside still has give, sometimes buttered while hot so the fat soaks in and waterproofs the crumb against the egg. The egg is cooked to a deliberate texture and placed, not stirred in: a sunny fried egg with a yolk meant to break and run, a soft folded omelet seasoned with dashi, or loose curds spooned on warm. Seasoning sits on top, salt and pepper, sometimes a thread of soy or a stripe of Japanese mayonnaise, occasionally a melt of cheese under a quick broil. A good one is eaten while the toast is still crisp and the yolk still loose, the two textures meeting in the same bite. A poor one waits too long: the toast steams soft under the egg, the yolk sets to chalk, and the contrast that justified the open face is gone.
The variations are mostly the choice of egg and what joins it. A runny fried egg over toast eats like a minimal breakfast plate; a thick dashi-seasoned rolled omelet pushes it toward kissaten, the old-style Japanese coffeehouse where egg dishes on toast are a fixture; cheese, ham, or a curry sauce sends it further toward a full yoshoku plate. Each of those readings is its own dish and deserves its own article rather than being crowded in here.