Onsen tamago has similar appearance to a poached egg, but onsen tamago, literally a hot spring egg, is originally cooked at hot springs. Now the term onsen tamago is used for particular type of soft boiled eggs.
Properly done onsen tamago is very soft, but not runny. Control the water temperature between 140°F~160°F is crucial to make it right.
Most often it is served with the mixture of katsuo dashi, soy sauce, and mirin (you can use noodle soup base). But you can use this to make tamago kake gohan, instead of a raw egg, to avoid salmonella poisoning.
If you have a sous vide cooker, you can make onsen tamago very easily.