We see the word 'atonement' in Romans 3:25-26.
25 God presented Christ as a sacrifice of atonement, through the shedding of his blood—to be received by faith. He did this to demonstrate his righteousness, because in his forbearance he had left the sins committed beforehand unpunished— 26 he did it to demonstrate his righteousness at the present time, so as to be just and the one who justifies those who have faith in Jesus.
William Lane Craig writes:
"The word “atonement” is unique among theological terms, being a derivation, not from Greek or Latin, but from Middle English, namely, the phrase “at-onement,” designating a state of harmony. The closest New Testament (NT) word for atonement in this sense is katallagē or reconciliation, specifically reconciliation between God and man. Reconciliation is the overarching theme of the NT, and other important NT motifs such as the Kingdom of God, salvation, justification, and redemption are subservient to it. Atonement in this sense thus lies at the heart of the Christian faith."
- Craig, William Lane. The Atonement, p. 4.