A term used in Shi'i terminology, "hujja" means "proof [implied: proof of God]." It is usually used to refer to a single individual in any given human era who represents God's "proof" to humanity. The hujja is a prophet or an Imam who possess a relationship with God that is greater than anyone else. The Imam who is the hujja of his time functions as the ultimate mediator between God and humanity, giving the Imam the greatest precedence for interpretation of the Qur'an. As the mediator between God and humanity, the Imam is the only one who can properly resolve conflicting interpretations of the Qur'an's words, giving the Imam ultimate authority over divine knowledge. In Twelver shi'ism the title "hujja" is specifically applied to the Twelfth Imam who is currently in a period of hiding and is attributed with the tradition of using Shi'i hadith to guide the religious community. The word Imam and hujja do not necessarily refer to the same person because an Imam may not be hujja but may keep the title of Imam.
The Imam that is hujja is hujja for several different logical proofs that are supported by Shi'i Qur'an interpretation and Shi'i hadith. The first proof of an Imam who is hujja is presented by the Imam's role as mediator between God and humanity. The divine appointment of the Imam, according to Shi'i belief, was passed down from the Prophet Muhammad to 'Ali and his sons al-Hasan and al-Husayn, who passed the divine knowledge onto their sons and so on. Therefore, it is only those who are members of the Prophet's family line that possess the divine knowledge from God and therefore are hujja.
The second proof an Imam is hujja is shown by the inner guidance the Imam provides for mankind "for he is a channel of divine grace which comes to him inwardly from the suprasensible realm". The Imam with his extensive knowledge of the different levels of human behavior and spiritual faith, is able to influence others thoughts and inner beings to help aid them in the refinement of their souls and inward journey. The Imam's divine guidance from God gives him the ability to lead and influence, which is why the Imam is hujja (proof of God).
The third proof an Imam is hujja is based on the Imam's immunity from the pollution of human sin. The Imam is such a divine spiritual figure he is free from committing human error or misinterpreting the Qur'an, which would otherwise lead to human error and sin. For a man who does commit sin is not fit to lead for he can spread sin and is therefore denied the rank of Imam and therefore cannot possess hujja. Only those who are free from error can be considered divinely touched and therefore are hujja and entitled to the Imamate.