An egg shaker is a hand percussion

Hand percussion

Hand percussion is a term used to indicate a percussion instrument of any type from any culture that is held in the hand. They can be made from wood, metal or plastic and are usually either shaken, scraped or tapped with fingers or a stick...

 instrument, in the idiophone

Idiophone

An idiophone is any musical instrument which creates sound primarily by way of the instrument's vibrating, without the use of strings or membranes. It is the first of the four main divisions in the original Hornbostel-Sachs scheme of musical instrument classification...

 category, that makes a noise when shaken, usually by hand. Functionally it is similar to a maraca. Typically the outer casing or container is oval or egg-shaped. It is partially full of small, loose objects, such as seeds or beads, which create the percussive sounds as they collide, both with each other and with the inside surface of the container.