IPA: /ˌvælɪˈdɪkʃən/
KK: /væləˈdɪkʃən/
A speech or statement that is given to say goodbye, often at the end of a formal event or letter.
The valediction at the graduation ceremony moved everyone to tears.
Valediction is formed from "vale" (from Latin "vale", meaning farewell) and "dictio" (from Latin "dictio", meaning saying or speech). The word "valediction" refers to the act of saying farewell or a farewell speech.
Think of saying 'farewell' ('vale') as part of a 'speech' ('dictio') — that's what valediction means.