"The Atonement"
is...
the wiping away of our sins
by the death of Jesus
on the cross.
A neat tip to help remember what  'atonement' means, is to remember that before you were
born again, your sin separated you from
God;  but after you became born again,
God's forgiveness closed that
gulf of separation, allowing
you to now be, 
'AT  ONE-- ment'   
with the   
Lord!    

...For if, when we were enemies,
we were reconciled to God by the death of His Son,
much more, being reconciled,  we shall be saved by His life.  And not only so, but we also joy in God
through our Lord Jesus Christ,
by whom
we have now received the atonement.

Romans 5:11