SSAS: Dimension Relationships in Cubes

“Dimension relationship” refers to the direct or indirect relationships between dimension and its measure groups in a Cube.

Regular Refers to a standard relationship, when a Key column in the dimension is directly joined to fact table.
Reference When a Key column in the dimension is indirectly joined to fact table by referencing another dimension.
Fact / Degenerate Dimensions constructed from attribute columns in fact tables than from attribute columns in dimension tables.
Many-to-Many One dimension is associated with multiple facts

Read more: https://docs.microsoft.com/en-us/sql/analysis-services/multidimensional-models-olap-logical-cube-objects/dimension-relationships?view=sql-server-2017

Note: My study notes