der Anfangsbuchstabe
initial letter
The first letter of a word, a name, or a sentence.
Der Anfangsbuchstabe von „Apfel“ ist „A“.
The initial letter of 'Apfel' (apple) is 'A'.

initial letter
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initial letter
The first letter of a word, a name, or a sentence.
Der Anfangsbuchstabe von „Apfel“ ist „A“.
The initial letter of 'Apfel' (apple) is 'A'.

The word is a clear compound of 'Anfang' (beginning) and 'Buchstabe' (letter). The 's' in the middle is a linking -s that connects the two words. The meaning is literal: the letter at the beginning.
Nomen erkennt man oft daran, dass ihr Anfangsbuchstabe großgeschrieben wird.
Nouns can often be recognized by the fact that their initial letter is capitalized.
Common verbs used with 'Anfangsbuchstabe' (initial letter) are 'großschreiben' (to capitalize) or 'kleinschreiben' (to write in lowercase). It is also often used in contexts like 'nach dem Anfangsbuchstaben sortieren' (to sort by initial letter).
Bitte sortieren Sie die Liste der Teilnehmer nach dem Anfangsbuchstaben des Nachnamens.
Please sort the list of participants by the initial letter of their last name.
To say what the letter is the first letter of, you usually use the structure 'der Anfangsbuchstabe von' (the initial letter of) followed by a noun in the dative.
Was ist der Anfangsbuchstabe von deinem Vornamen?
What is the initial letter of your first name?
initial
'die Initiale' and 'der Anfangsbuchstabe' often mean the same thing: the first letter of a name or word. 'Anfangsbuchstabe' is very direct and easy to understand in everyday language. 'Initiale' sounds somewhat more formal and is used especially often with names, for example a person's initials. In many situations, you can use both words. If you want to speak especially simply, 'Anfangsbuchstabe' is usually the clearer choice.

Der Anfangsbuchstabe ihres Nachnamens ist schwer zu lesen.
The initial letter of her last name is hard to read.
Karim hat seine Initialen auf das Formular geschrieben.
Karim wrote his initials on the form.