schriftlich
written
In written form, as opposed to oral. This can be on paper or in digital form, e.g. by email.
Die Kündigung des Vertrags muss schriftlich erfolgen.
The termination of the contract must be made in writing.

written
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written
In written form, as opposed to oral. This can be on paper or in digital form, e.g. by email.
Die Kündigung des Vertrags muss schriftlich erfolgen.
The termination of the contract must be made in writing.

The word 'schriftlich' (written/in writing) is derived from the noun 'die Schrift' (writing/script). The suffix '-lich' turns it into an adjective and means 'in the manner of writing' or 'in written form'.
Bitte senden Sie uns Ihre Anfrage schriftlich zu.
Please send us your inquiry in writing.
'Schriftlich' (in writing) is very often used as an adverb and then describes how an action is carried out. In this case, the word is not declined.
Sie müssen den Vertrag schriftlich kündigen.
You must terminate the contract in writing.
'Schriftlich' (written/in writing) is often used in formal and official contexts. Common combinations are 'schriftliche Prüfung' (written exam), 'schriftlicher Antrag' (written application/request), 'etwas schriftlich festhalten' (to put something in writing), or 'eine schriftliche Bestätigung' (a written confirmation).
Für die Bewerbung ist ein schriftlicher Lebenslauf erforderlich.
A written résumé is required for the application.
black
'schwarz auf weiß' and 'schriftlich' both have to do with written form. 'schriftlich' is the normal word when something is written, either on paper or digitally. 'schwarz auf weiß' is a fixed expression and means more: you have something as clear written proof or a binding confirmation. Learners may confuse the words because both refer to something that is not spoken. If you only mean the form, use 'schriftlich'. If you want to emphasize that something is verifiably written down, 'schwarz auf weiß' fits.

Bitte schicken Sie die Anmeldung schriftlich per E-Mail.
Please send the registration in writing by email.
Ich möchte die Zusage schwarz auf weiß haben, bevor ich kündige.
I want to have the confirmation in black and white before I quit.