Clausen

Clausen is a Danish patronymic surname, literally meaning "child of Claus", Claus being a German form of the Greek Νικόλαος, Nikolaos, (cf. Nicholas), used in Denmark at least since the 16th century. The spelling variant Klausen has identical pronunciation (as does the often interchangeable Claussen). 
The two variants are number 34 and 85 on the top100 of surnames in Denmark. Occurrences of Clausen/Klausen as a surname outside Denmark and Schleswig-Holstein are due to immigration. Immigrants to English-speaking countries sometimes changed the spelling to Clauson.