consent; giving one's word
A single word of agreement or promise. Literary and somewhat old-fashioned; in modern Japanese, 承諾 or 同意 are more common.
He gave his consent.
There is also the proverb 'a single promise is worth a thousand pieces of gold.'
Compound of 一 (one) and 諾 (consent, agreement). The exact historical derivation is uncertain, but it follows a classical Chinese pattern of using 一 to emphasize a single, decisive act.