Nitsume is a Japanese sauce used with eel, octopus and shrimp.
There used to be many variations of it. One older way of making it was simmering eels in a pot of water for several months until all that was left was a thick glaze.
Now, the process for making it has become pretty much standardized. It is made from eel broth, mirin, soy sauce and sugar, reduced by about one-third through slow simmering.
Language Notes
In Japanase, “Nitsume” means “boiling down.”