A column (usually granular) is called as critical column which changes the values over a period of time.
For example, there is a customer by name ?Anirudh? who resided in Bangalore for 4 years and shifted to Pune. Being in Bangalore, he purchased Rs. 30 Lakhs worth of purchases. Now the change is the CITY in the data warehouse and the purchases now will shown in the city Pune only. This kind of process makes data warehouse inconsistent. In this example, the CITY is the critical column. Surrogate key can be used as a solution for this.
Its a column whose values changes overa period of time.
EX: Person city -- abc warehouse will keeping track of his travelling route, when the users changes his city the data becomes inconsistent since the city has changed, route will be showed under the new city