A solemn affirmation is a serious promise or vow made by a person. The three crossword solutions for this term are 'OATH' (4 letters), 'AMEN' (4 letters), and 'Ido' (3 letters). An OATH is a formal, binding declaration of truthfulness or commitment, often made in the presence of a witness or witnesses. AMEN is a Hebrew word meaning 'so be it' or 'TRULY, ' and is used to express agreement or acceptance. Lastly, 'Ido' is a shortened form of the phrase 'I do, ' which is used in marriage ceremonies to signify agreement to the union.