Dots over the vowel in Köln (6)
I believe the answer is:
umlaut
(Other definitions for umlaut that I've seen before include "Two dots put over a vowel in German" , "Diacritical sign" , "Mark over some German letters" , "German accent" , "Diacritical mark (two dots) placed over a vowel in German to indicate a change in sound" .)