A formal or old-fashioned term for a spoken promise or pledge. Rare in modern conversation; more common equivalents are 約束 or 誓約.
He kept his word and repaid the debt without fail.
Compound of 言葉 (ことば, 'word') and 質 (じち, 'pledge' or 'pawn'). The exact historical derivation is uncertain, but the word is now rare.