What is atonement?

The 1st target of atonement was the priest family, Leviticus 16:

6 “Aaron shall offer the bull as a sin offering for himself and shall make atonement for himself and for his house.

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Strong's Exhaustive Concordance

appease, make an atonement, cleanse, disannul, forgive, be merciful, pacify, pardon, A primitive root; to cover (specifically with bitumen); figuratively, to expiate or condone, to placate or cancel -- appease, make (an atonement, cleanse, disannul, forgive, be merciful, pacify, pardon, purge (away), put off, (make) reconcile(-liation).

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Atonement was the ritual of making things right with God.

The 2nd target was the Holy Place, Leviticus 16:

15“Then he shall kill the goat of the sin offering that is for the people and bring its blood inside the veil and do with its blood as he did with the blood of the bull, sprinkling it over the mercy seat and in front of the mercy seat. 16a Thus he shall make atonement for the Holy Place

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The 3rd target was the Israelites, Leviticus 16:

17 No one may be in the tent of meeting from the time he enters to make atonement in the Holy Place until he comes out and has made atonement for himself and for his house and for all the assembly of Israel.

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It was a yearly cleansing ritual, Leviticus 16:

29 “And it shall be a statute to you forever that in the seventh month, on the tenth day of the month, you shall afflict yourselves and shall do no work, either the native or the stranger who sojourns among you. 30 For on this day shall atonement be made for you to cleanse you. You shall be clean before the LORD from all your sins. ... 33 He shall make atonement for the holy sanctuary, and he shall make atonement for the tent of meeting and for the altar, and he shall make atonement for the priests and for all the people of the assembly. 34 And this shall be a statute forever for you, that atonement may be made for the people of Israel once in the year because of all their sins.”

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That's a nice definition of OT atonement. It cleansed all Israelites from all their sins on this day. However, that was only a shadow type. The ultimate true type is found in Christ, Hebrews 10:

1 For since the law has but a shadow of the good things to come instead of the true form of these realities, it can never, by the same sacrifices that are continually offered every year, make perfect those who draw near. ...

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Jesus died on the cross, as the Lamb of God, the perfect sacrifice.

10 And by that will we have been sanctified through the offering of the body of Jesus Christ once for all.

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Atonement was the ritual of making amends or seeking forgiveness for wrongdoing or sin. In the OT, the Israelites went through this ritual yearly. In the NT, Christ fulfilled it once and for all people and all time.