Today, is broadly used in any context related to spirit possession, and it can be anything or anyone. It's also commonly used outside the contexts of Japanese _shinto_. It can be a piece of paper, a sword, a tree, a rock, a doll, an animal or a human being. I think it's perfectly fine to use to describe some ritual item used by a Voodoo priestess, for example.
According to sources, the word was invented in the early 20th century by an ethnologist Shinobu Orikuchi to explain a certain aspect of religious belief of Japanese people. It's not a traditional _shinto_ term, and by its nature, the word will not determine what a concrete must look like.