What Are Oat Berries?

In one of my herb books, there was a tea for bladder incontinence that had oat berries as one of the ingredients. I can not find any information on what an oat berry is, what part of the plant or if it even is a part of the oat plant as we know it.

The oat berry is the inner grain after removing the chaff or husk. It refers to the whole, unrolled form of the grain.