B.B. Warfield explains the three imputations of Christian theology

"In the developed theology thus brought into the possession of the Church [during the Reformation], three several acts of imputation were established and expounded. These are the imputation of Adam's sin to his posterity; the imputation of the sins of His people to the Redeemer; the imputation of the righteousness of Christ to His people. ... But in each and every case alike imputation itself is simply the act of setting to one's account; and the act of setting to one's account is in itself the same act whether the thing set to his account stands on the credit or debit side of the account, and whatever may be the ground in equity on which it is set to his account. That the sin of Adam was so set to the account of his descendants that they have actually shared in the penalty which was threatened to it; and that the sins of His people were so set to the account of our Lord that He bore them in His own body on the tree, and His merits are so set to their account that by His stripes they are healed, the entirety of historical orthodox Christianity unites in affirming." (B.B. Warfield, "Imputation," from "The Works of Benjamin B. Warfield," vol. IX, pp. 302-303, published by Baker)