Mostly the word used is
decorated initial letter.
link 1. link 2. reversed dictionary gives for decorated initial letter as number one: illuminate. ( See twosleepy )
one look reversed Also this:
majuscule SYLLABICATION: ma·jus·cule
PRONUNCIATION: m-jskyl, mj-skyl
NOUN: A large letter, either capital or uncial, used in writing or printing.
ETYMOLOGY: French, from Latin miusculus, somewhat larger, diminutive of mior, greater. See meg- in Appendix I.
Or else this:
rubric :2. A part of a manuscript or book, such as a title, heading, or initial letter, that appears in decorative red lettering or is otherwise distinguished from the rest of the text.