Some exegetes have said that the word Imamah means prophethood and apostleship and others say that it means successorship and they have offered such interpretations from time to time.
A Prophet receives news from God while a Messenger coveys the message of God to the people. Successorship means becoming a successor to the Prophet. All of this is different from the meaning of Imamah.
This debasing of the word Imamah has degraded the sublime meanings of the Qur'anic words in the minds of people. The difference between Imamah and the above mentioned words (prophethood, messengership, successorship) is not only verbal but the realities behind these titles that differ from one another.
Imamah implies that a man has an intrinsic quality because of which people should follow him faithfully making their words and deeds to conform with his words and deeds. A good example is Prophet Abraham who was given Imamah at old age while he was already a prophet and a messenger as per verse 124 of the second Chapter of the Qur'an: And (remember) when His Lord tried Abraham with certain words, then he fulfilled them: He said: Surely I am going to make you an Imam for men. (Abraham) Said: And of my offspring? He said: My covenant will not include the unjust. It would not make sense to tell Abraham that God was going to make him a prophet or a messenger when he was already a prophet and a messenger.
What we find in the Qur'an is that whenever Imamah is mentioned, it mentions guidance by its side - as though God was using the latter to explain the former. God says in the story of Abraham: And We gave him Isaac and Jacob, a son's son, and We made (them) all good. And We made them Imams who guided (people) by Our command... (21:72-73); and He says in another place: And We made of them Imams to guide by Our command when they were patient, and they were certain of Our signs (32:24). Here the Imamah is defined in terms of guidance and then further qualified with "by Our command".
Clearly, the Imamah does not mean any type of guidance; it is a guidance that emanates from the command of God. The reality of this command is mentioned in these verses: His command, when He intends anything, is only to say to it: Be, so it is. Therefore glory be to Him in Whose hand is the kingdom of all things... (36:82-83); And Our command is but one, as the twinkling of an eye (54:50).
In conclusion, an Imam is a leader who guides by a Divine Command which is his intrinsic quality. The Imamah in its esoteric sense is al-Wilayah (guardianship) over the people in their actions and affairs; its guidance entails conveying them to the final destination by the command of God. I
Reference: Al-Mizan English Volume 2, pp. 71-79, first edition, WOFIS, 1986.