The Collaborative International Dictionary of English v.0.48:

lowercase \low"er*case`\, Lower-case \Low"er-case`\, a. (Print.)
   Pertaining to, or kept in, the lower case; -- used to denote
   the small letters, in distinction from capitals and small
   capitals; as, lowercase letters a and b and c etc..
   Contrasted with uppercase. See the Note under 1st Case,
   n., 3.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lowercase
    adj 1: relating to small (not capitalized) letters that were
           kept in the lower half of a compositor's type case;
           "lowercase letters; a and b and c etc" [ant: uppercase]
    n 1: the characters that were once kept in bottom half of a
         compositor's type case [syn: small letter, lowercase,
         lower-case letter, minuscule] [ant: capital, capital
         letter, majuscule, upper-case letter, uppercase]