Simply, to credit or reckon. Through Adam, the guilt of sin is imputed to all men; through Christ, righteousness is imputed to believers (Rom 5:12-21). On the cross, Christ exchanged his righteousness for man’s sinfulness (2 Cor 5:21) by means of imputation. The sins of believers were imputed (credited) to Christ on the cross, and the righteousness that belonged to Jesus Christ was imputed (credited) to believers. Thus, believers possess an “alien” righteousness and can stand before a righteous God.
(Source: WHI terms-to-learn “Imputation”)