al-Kufr (kufr), is interpreted in four senses:
1. al-Kufr is used in the sense of kufr towards Allah's tawhid (the oneness of God), that is, to deny Him; as in the following verses:
Indeed, those who kufr (that is, those who deny Allah's tawhid / that He is the One and only God)—it is the same for them whether you warn them or do not warn them—they will not believe. (al-Baqara 2:6)
Those who kufr {that is, who deny Allah's tawhid} and hinder [others] from the way of Allah... (Muhammad 47:1)
There are many similar statements.
2. al-Kufr is used in the sense of denying the hujja/dalil (proof or evidence); as in the following verses:
And when there came to them that which they recognized, they kufr (that is, they recognized it, but they denied it). (al-Baqara 2:89)
Those to whom We gave the Scripture [the Jews and Christians] recognize him as they recognize their [own] sons (that is, they recognize the Prophet as they recognize their sons; for his attributes are found in the Torah with them). Those who have lost themselves will not believe. (al-Anʿām 6:20)
Those to whom We gave the Scripture recognize it (that is, that the Kaʿba is the qibla) as they recognize their [own] sons. But indeed, a party of them conceal the truth while they know [it]. (al-Baqara 2:146)
Pilgrimage to the House [Kaʿba] is a duty owed to Allah by all people who are able to undertake it. And whoever kufr (that is, whoever from the People of the Book or from other religions denies the obligation of pilgrimage to Allah's Sacred House and rejects its obligation), then indeed, Allah is free from need of the worlds (of the People of the Book and others). (Āl ʿImrān 3:97)
3. al-Kufr is used in the sense of kufrān al-niʿma (ingratitude for blessings); as in the following verses:
So be grateful to Me and do not kufr (that is, do not show kufr—ingratitude—for My blessing)! (al-Baqara 2:152)
(Solomon said): "[This is] so that He may test me whether I will show shukr (gratitude) or kufr (that is, whether I will show kufr—ingratitude—for the blessing)." (al-Naml 27:40)
Be grateful to Allah; and whoever shows shukr, it is for [the benefit of] his own soul; and whoever kufr {that is, shows kufr—ingratitude—for the blessing}, then indeed, Allah is Free of need and Praiseworthy. (Luqmān 31:12)
{Pharaoh said to Moses}: "So you did your deed which you did, and you are of the kuffār {that is, you are of the ungrateful; by this, he notes that he raised Moses as a child and did good to him, but Moses responded with ingratitude}." (al-Shuʿarāʾ 26:19)
There are many similar verses.
4. al-Kufr is used in the sense of being free from / distant from, to disassociate; as in the following verses:
{Abraham said to his father and his people}: "We have kufr towards you {that is, we have declared ourselves free from you / we have distanced ourselves from you}; and enmity and hatred have appeared between us and you forever." (al-Mumtaḥina 60:4)
Then, on the Day of Resurrection, some of you will kufr towards others (that is, some of you will declare yourselves distant from others). (al-ʿAnkabūt 29:25)
{Iblis will say to those who obeyed him}: "I had kufr towards your associating me [with Allah in obedience] before" {that is, I had declared myself distant from it}. (Ibrāhīm 14:22)
There are many similar statements.