The literal meaning of the term Kafir is 'he who conceals'. The cultivator, too, who hides the seeds in the soil is called a Kafir. To call the one who hides his accomplishments a Kafir was a practice that was in vogue even in the earliest Arabia. It can also be seen that the term Kafir has been used to aver to an ungrateful person.
The Qur'an, however, uses Kafir to refer to a denier of the truth, one who shows ingratitude, a disbeliever and the like not in any uncouth manner; rather the Qur'an uses this term to explain the nature of those it intends to expose. The singular form of the term - Kafir - as well as the forms Kafirun and Kuffar, which are the plural derivatives, have been repeatedly used in the Qur'an. In all such instances the Qur'an has sought to explain the character of those it mentions within the context. Look at a few of the verses of the Qur'an
:'Those who deny God and His apostles, and (those who) wish to separate God from His apostles, saying, 'We believe in some but reject others' : and (those who) wish to take a course midway. They are in truth (equally) the Kafirun; and We have prepared for the Kafirs a humiliating punishment.' (Qur'an 4:150,151)
'They recognize the favours of God; then they deny them; and most of them are Kafirun (i.e. deniers of the truth / ungrateful ones)' (16:83)
'It was We who revealed the Law (to Moses): therein was guidance and light. By its stand have been judged the Jews, by the prophets who bowed (as in Islam) to God's will, by the Rabbis and doctors of Law: for to them was entrusted the protection of God's Book, and they were witnesses thereto: therefore fear not men, but fear Me, and sell not My signs for a miserable price. If any do fail to judge by (the light of) what God hath revealed, they are (no better than) the Kafirun.' (5:47)
'Say : O ye Kafirun! I worship not that which ye worship. Nor will ye worship that which I worship. And I will not worship that which ye have been wont to worship, nor will ye worship that which I worship. To you be your way, and to me mine.' (109:1-6)
From these verses we understand that the Qur'an has used the term Kafir to refer to those who, in not acknowledging divine guidance, reject truth and show ingratitude to Allah, the Most Bounteous. Those who enjoy the blessings of the Lord Creator and yet abstain from acknowledging Allah, the Provider of all bounty are, indeed, Kafir. Those who deviate from the worship of Allah alone - the one thing that the Lord Creator, who provided for man all the means of his sustenance on earth, demanded of us - and pray unto the creations that are never to be worshipped, are Kafir. Those who do not acknowledge the messengers whom the Lord Creator had sent to illumine the path of guidance are Kafir. The one who does not regulate his life according to the religious scripture revealed, by the Lord Creator, as a criterion to judge between truth and falsehood, is a Kafir. It is the Kafir who have become enemies of the Truth who strive to blow out the divine light of guidance.