The three-letter Arabic root (خلف) KH-L-F has many meaning, one of which indicates making one a successor after another, to give reign of power and ascendancy after a proceeding ruler or group’s reign, and all the responsibilities associated with power.
The word ‘Caliph’ is used in English in lieu of Arabic word Khaleefah [singular (خليفة), plural Khulafa`a (خلفاء)], which means the leader and ruler coming one after another, and following the preceding lead.
A Khaleefah is sometimes called a vicegerent and deputy of a higher authority, executing the higher authority’s commands that are also his commands by loyal allegiance to that higher authority.
This is one of the interpretations given for the Prophet and father of mankind Adam, peace be upon him, and for the Prophet Da`ud (David), may Allah’s peace be upon him, both of whom are called a Khaleefah, in the sense that they represent the execution of the law and rule of Allah the Exalted Almighty God of all creation, on their limited domain on themselves and on earth.
Allah the Exalted said:
وَإِذْ قَالَ رَبُّكَ لِلْمَلَائِكَةِ إِنِّي جَاعِلٌ فِي الْأَرْضِ خَلِيفَةً
قَالُوا أَتَجْعَلُ فِيهَا مَنْ يُفْسِدُ فِيهَا وَيَسْفِكُ الدِّمَاءَ
وَنَحْنُ نُسَبِّحُ بِحَمْدِكَ وَنُقَدِّسُ لَكَ قَالَ إِنِّي أَعْلَمُ مَا لَا تَعْلَمُونَ
“And when your Lord said to the angels, ‘Indeed, I will make upon the earth a Caliph,’ they said, ‘Will You place upon it one who causes corruption therein and sheds blood even though we glorify You and sanctify You?” He said, ‘Indeed I know that which you do not know.’ (2:30)
There are two sayings of the companions and the commentators about the word Caliph “Khaleefah” here in this verse.
One saying is that he is a “Vicegerent” in the sense that Allah designated him the duty to observe and to establish Allah’s laws.
The other saying is that he is a “Successor” of those proceeding, and thus man is succeeding generations of those proceeding, each generation following the other.
[See the commentaries of the Qur`an which mention these two reported sayings among the companions and followers after them, like at-Tabari 1/200, Ibn Katheer 1/70 and Ibn Jauzi 1/60 and others.]