The color temperature of daylight which is 5,400K on the color temperature scale (it does vary during the day, being higher at noon and lower in the earlier and later parts of the day). Color film for outdoor shooting is balanced for daylight, otherwise the image would appear blue in hue. If daylight balanced film is used indoors without a correction filter the image will have an orange hue.
Glossary of Film Terms - http://homepage.newschool.edu/~schlemoj/film_courses/glossary_of_film_terms/
reprinted with permission