WordNet (r) 3.0 (2006):

upper-case letter
    n 1: one of the large alphabetic characters used as the first
         letter in writing or printing proper names and sometimes
         for emphasis; "printers once kept the type for capitals and
         for small letters in separate cases; capitals were kept in
         the upper half of the type case and so became known as
         upper-case letters" [syn: capital, capital letter,
         uppercase, upper-case letter, majuscule] [ant:
         lower-case letter, lowercase, minuscule, small
         letter]