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.