capital letter

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A letter written or printed in a size larger than and often in a form differing from its corresponding lowercase letter; an uppercase letter.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An upper-case letter (A, B, C, ... as opposed to a, b, c, ...), used for emphasis, for starting sentences and proper names, etc.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun one of the large alphabetic characters used as the first letter in writing or printing proper names and sometimes for emphasis