adjective
Chronologically first, early; of or pertaining to the beginning, cause or origin.
Spatially first, placed at the beginning, in the first position; especially said of the first letter of a word.
noun
The first letter of a word or a name, especially of a person's full name (their initials).
(typography, calligraphy) A distinguished initial letter of a chapter or section of a document.
(phonology) onset, part of a syllable that precedes the syllable nucleus in phonetics and phonology.
verb
(transitive) To sign one's initial(s), as an abbreviated signature.