Tendon is a bowl of rice topped with pieces of tenpura. Tendon served in a jubako (tiered food box) is called Tenju.
Tendon, as it's commonly called today, is an abbreviation for "Tenpura Donburi."
The most common way of making Tendon is to top a bowl of rice with a variety of tenpura and pour salty-sweet sauce over it; however, there are other methods, including flavoring the tenpura instead of pouring sauce on it or just sprinkling a bit of salt over the tenpura. The sauce is made by mixing soup stock, soy sauce, sugar and mirin, which is a sweet rice wine used for cooking.
Tendon is available at tenpura specialty restaurants and soba shops. Of all the donburi varieties, Tendon is considered to be the top.
It is said that Tendon was first served at a restaurant called Sansada (Asakusa, Taito Ward, Tokyo; founded in 1838), where a bowl of rice was topped with deep-fried diced shrimp, whitefish and vegetables with sauce poured over it.
There is very little restriction on the type of food that can be deep-fried as tenpura, including seafood such as shrimp, squid, sea eel and sand borer, which are accompanied by vegetables such as small, deep-fried sweet peppers, pumpkin and sweet potatoes. Tendon is sometimes distinguished by the main ingredient; ebi-tendon is topped with shrimp, and anago-tendon is topped with sea eel.
Shojin Tendon (vegetarian tendon)
This is tendon topped with vegetable tenpura. It is sometimes called Yasai (vegetable) Tendon.
As the name suggests, this is Tendon topped with kakiage (deep-fried diced shrimp, whitefish and vegetables).
Tentama-don (or Tentoji-don)
A bowl of rice topped with tenpura flavored with salty-sweet sauce and beaten egg poured over it.
Tendon as standup-comic jargon
In standup comedy acts, when the same thing is repeated twice or more for comic effect, it's called "Tendon."
It came from the fact that Tendon is usually topped with two pieces of shrimp.