first letter of a word; capital letter (at the start of a word or sentence)
initials (of one's name)
NGOとは頭文字を並べた略称だ。
NGO is an abbreviation formed by arranging initials.
文章の頭文字を大文字にするのは英語の決まりだ。
Capitalizing the first letter of a sentence is a rule in English.
名前の頭文字を縫い込んだハンカチをプレゼントした。
I gave a handkerchief embroidered with the initials of a name as a gift.