capital letter

noun

a letter of the alphabet that usually differs from its corresponding lowercase letter in form and height, as A, B, Q, and R as distinguished from a, b, q, and r: used as the initial letter of a proper name, the first word of a sentence, etc.

Also called capital.

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Examples from the Web for capital letter

If the capital-letter be retained where a prefix is put to a proper name, the hyphen is obviously necessary.
"Stops"
Paul Allardyce

late 14c.; see capital (adj.). So called because it is at the "head" of a sentence or word.