What is degenerate dimensions)

What is degenerate dimensions? The definition of degenerate dimensions is Ralph.
Kimball proposed. Generally, the foreign keys in a fact table correspond to a dimension table, and the dimension information is mainly stored in the dimension table. However, the degraded dimension is only a column in the fact table.
No dimension table is associated with the column. For example, the invoice number and serial number.

So what is the role of a degraded dimension?
1. Degraded dimensions have various operations for common dimensions, such as rolling, slicing, and chunking.
2. If there is a degraded dimension, the ETL process will become easier.
3. It can make group by and other operations faster.