If ever an example was needed for the value in understanding the original languages of scripture then the word 'atonement' would be a good candidate for providing it. In the Old Testament it is always a translation of the verb kâphar (H3722) or the plural masculine noun kippûr (H3725) which is a derivative of the verb. The latter has obviously moved into regular parlance as a component of 'Yom Kippur' which is the Day of Atonement, still celebrated by Jews today and also the title of war.
One of the more interesting features of the word kâphar is that the first use of it in Gen 6:14 is uniquely translated as 'pitch' whereas it is generally translated as atonement (73), purge(9), reconcile (6), forgiven(3) or pacify(2). Brown, Driver and Briggs gives the reason for this oddity; the verb really means to cover over. Thus Noah was told to cover over the ark with bitumen, Jacob sought to cover over his differences with Esau and the Jews were then told to cover over their sins with various sacrifices.
We therefore see that the Sunday school definition of atonement as at-one-ment is rather dubious. The pitch did not make the ark at one with the sea, Jacob was not at one with Esau and whilst the relationship between God and Israel was intimate the repeated need for atonement shows that it wasn't total. I suspect that the best modern expression would be that the sins of Israel had been swept under the carpet.
Strictly speaking 'atonement' is not a New Testament word. It is true that it appears in Rom 5:11 in the KJV where it is a translation of katallagē, but this word is usually translated reconcile in the KJV and is rendered 'reconciliation' in Rom 5:11 in most translations and according to Vine should be rendered 'reconciliation' always.
It is however interesting to contrast the meaning of katallagē with the Old Testament kâphar. The definition of katallagē according to Thayer:
1a) of the business of money changers, exchanging equivalent values
2) adjustment of a difference, reconciliation, restoration to favour
2a) in the NT of the restoration of the favour of God to sinners that repent and put their trust in the expiatory death of Chris
Thus katallagē means to exchange two things of like value or to bring two things up to equal value. We therefore see that in contrast to kâphar with katallagē nothing is hidden or covered but the necessary exchange has been completed to bring reconciliation.
In conclusion we have seen that atonement has a simple but slightly surprising definition: to cover over. This can be true literally of smearing tar upon a boat or figuratively of smearing animal blood over sin. In the New Testament this concept is replaced with the notion of reconciliation: the complete equalization of the debt and the payment.