Function: adjective
Date: 1839
1 : first in rank, importance, or influence : FOREMOST <cancer is the country's number one killer> ― often written North 1
2 : of highest or of high quality
NUMBER ONE
Meaning of NUMBER ONE in English
Merriam Webster Collegiate English Dictionary. Merriam Webster - Энциклопедический словарь английского языка. 2012