What is Eye of a needle?

The term "eye of a needle" is used as a metaphor for a very narrow opening. It occurs several times throughout the Talmud. The New Testament quotes Jesus as saying that "it is easier for a camel to go through the eye of a needle than for a rich man to enter the kingdom of God". It also appears in the Qur'an 7:40, "Indeed, those who deny Our verses and are arrogant toward them – the gates of Heaven will not be opened for them, nor will they enter Paradise until a camel enters into the eye of a needle. And thus do We recompense the criminals."