What is the difference between sushi, sashimi, and nigiri?

(Originally answered in Quora.com)

What is Sushi?（寿司、鮨）

By definition, Sushi is Sushi Rice plus ingredients. Sushi rice is Vinegared Rice, usually seasoned with Rice Vinegar, Sugar and Salt. Ingredients can be anything - fish, vegetables, and meats. The word Sushi comes from Su(酢vinegar) Meshi(飯rice) in Japanese. Sushi comes in many forms and varieties. You can see them here: How many types of sushi are there?

What Sushi is NOT

Sushi does not mean Raw Fish. Sashimi is not Sushi. Nigiri does not automatically mean Sushi, though, many Japanese assume it to be Sushi when used in the conversation.

What is Sashimi?（刺身）

Sliced meat. When Japanese say Sashimi, they usually assume it to be “raw fish” Sashimi. Other varieties of Sashimi are Chicken Sashimi, Yuba(bean curd) Sashimi, Konnyaku(a type of potato) Sashimi.

What Sashimi is NOT

Raw fish is not Sashimi. Sashimi is not always raw fish.

What is Nigiri?（握り）

A type of Sushi. The correct term is Nigiri-Zushi (握り寿司). Neta (ingredients) on top of small Sushi Rice. Nigiri means to squeeze, to grab in Japanese, referring to the movement a Chef makes to form Neta and Shari together. Neta on top of the Sushi Rice can be fish, vegetables or meats, where fish being the most widely served and consumed.

Gunkan, battleship is one type of Nigiri (far right, bottom raw, is Ikura Gunkan).

What Nigiri is NOT

Nigiri is not Sashimi. Nigiri is not raw fish. Nigiri is not always raw fish. Nigiri is not always fish.

Tamago, the egg is another type of Nigiri