What is the meaning of "dimension key"?
A dimension key is the primary key of a dimension table.

In the example below, the "Time_key" column is the dimension/primary key of the Time dimension. This column will be used to join the dimension with fact tables, just like any other key.



in https://www.kimballgroup.com/1997/07/its-time-for-time/

A degenerate dimension is, however, a different and slightly more advanced concept to model. In the degenerate dim case, a fact table contains a foreign key that, by design, does not link to a dimension.