Introduced on day 7
Slowly changing dimension, type 2
A dimension table that keeps one row per version of an entity's attributes, with valid-from and valid-to dates, so historical facts join to the attributes valid at the time.
In detail
Type 1 overwrites attributes and loses history; type 2 preserves it. Joining old facts to the current dimension version silently rewrites history (a route reclassified today changes last year's market totals).