A compound word - Koh + Noor. Koh means mountain and Noor means light. Kohinoor means "Mountain of Light". The 191 carat Kohinoor diamond has a long history. It was mined at Kolloor near the Krishnaa River. Its earliest reference is found with Alaauddeen Khilajee (1296-1316 AD), when he acquired it from the king of Maalavaa (Raajsthaan) in 1306 AD. From this time up to the time of Baabar its whereabouts are not known

In the "Baabar-naamaa", the first Mugal emperor relates that how he, after the victory of the 1st Paaneepat battle on April 21, 1526 AD, Baabar sent his son Humaayoon to Aagaraa to secure the treasure while he himself marched into Dehlee. Baabar records that "In Sultaan Ibraaheem Lodee's defeat (April 20, 1526), the king of Gwaalior Vikramaajit had gone to Hell. When Humaayoon reached Aagaraa, the king's family was planning to flee, but Humaayoon would not allow them to go. They made a voluntary offering of a mass of jewels and valuables, amongst which was the famous diamond which Alaauddeen Khilajee must have brought." This was one of the stories of the diamond's origin near Golkundaa in Deccan. Baabar went on, "The diamond's reputation is such that every appraiser has estimated its value 2 1/2 day's food for the whole world. Humaayoon offered it to me when I arrived at Aagaraa, I just gave it him back."

Humaayoon kept it for some time. Then Sher Shaah Sooree defeated him on May 17, 1540 AD. And Humaayoon then wandered for some 15 years. During this period he came to Afgaanistaan and he presented this diamond to its Shaah in 1544. The Shiyaa Muslims of Daccan, Ahmadnagar, Golkundaa and Beejaapur regarded Afgaanistaan Shaah as their head, so he sent that diamond to Bahraam Nizaam Shaah of Ahmadnagar, Daccan. It remained in Ahmadnagar and Golkundaa for 109 years - from 1547-1656. In 1656 Golkundaa's former Prime Minister Mr Jumlaa presented it to Shaah Jahaan. After he was deposed by his third son Aurangzeb in 1658, this diamond came in the hands of Aurangzeb. Muhammad Shaah Rangeelaa (1719-1748 AD) used to carry this diamond in his turban

Then the  Persian King Naadir Shaah invaded India in January 1739 and defeated Mugal King Muhammad Shaah Rangeelaa (1719-1748 AD) in the Battle of Karnaal, Panjaab, near Dehlee. Thus he got possession of Kohinoor Heeraa from that Mugal King on May 1, 1739. When he first saw the diamond, he was so dazzled by its size and beauty that he exclaimed "Kohinoor" which means "Mountain of Light" and that is how this name was given to that diamond. Before that nobody had heard this name for the diamond.

After Naadir Shaah of Dehlee was assassinated in 1747, it was passed on to Ahamad Shaah Durraanee in 1739 AD. Then his descendents were forced to hand it over to Ranjeet Sinh. But when Sikh were finally defeated by British in 1849 AD, the jewel was taken to London to Queen Victoria. It was presented to Queen Victoria in 1850 by East India Company to mark its 250th anniversary. It was re-cut in 1852 AD in London and it now weighs 108.93 carats, losing its 43% of its original weight. It was set in Queen Elizaabeth's, the Queen Mother, state crown for King George VI's coronation in 1937.

During its life time, it traveled through four countries - India, Persia, Afgaanistaan, and finally to England. It remained in possession of Mugals for 101 years, Persians for 11 years, then in India for 109 years, Kaabul and Kandhaar in Afgaanistaan for 66 years, and Sikhs in Laahaur for 36 years. British have it now almost for the last 160 years. Now it can be seen in Tower of London, England.

About its Origin
The origin of the diamond is unclear. According to some sources, the Kohinoor was originally found more than 5,000 years ago and is mentioned in Sanskrit writings under the name of Syamantak Mani. Hindu believe that Krishn Himself got it from Jaambvant who married his daughter Jaambvatee also to Him. The legend says that it was stolen from Krishn while He was sleeping. Another source says that it was found in a riverbed in 3200 BC. But historical evidence say that it was originated in Golkundaa's diamond mines, in Haidaraabaad in Aandhra Pradesh. Until 1730 AD Golkundaa's mines were the only source of diamonds, when diamonds were discovered in Brazil. [from Wikipedia]

[Wikipedia person has forgotten to write that, if this is the same Mani, means Syaamanak Mani, then "According to Bhaagvat Puraan, 10/56, originally it was with Satraajit who got it from Soorya Dev. It used to give 8 Bhaar gold everyday and had a lots of good effects. One day his brother took it from him, wore it and went to a forest when he went for hunting. A lion killed him there, and took that Mani away. The lion was about to enter his cave that Jaambvant saw him carrying that Mani, he killed him and took that Mani to his home. He gave that Mani to his child to play with. Since Krishn asked Satraajit to give that Mani to King Ugrasen, and he did not give it, he suspected that Krishn had stolen it. Krishn had to find it to remove this blame, so He went out and found it in Jaambvaan's cave - the child was playing with it. He could get it only after 28 days of fight with Jaambvaan along with his daughter Jaambvatee. He gave that Mani back to Satraajit, told the whole story and cleared Himself from the blame."]

The Khilajee Dynasty ended in 1320, and Gayaasuddeen Tugalaq ascended the throne in 1320. He sent his son Ulug Khaan in 1323 to defeat Prataaprudra of Waarangal. He defeated him and brought much gold, treasure, ivory etc on camels and horses along with this diamond. From then on it passed through the hands of the rulers Dehlee Saltanat, finally passing to Baabar.

The first confirmed note mentioning the Kohinoor by an identifiable name dates from 1526, when Baabar mentioned it in his Baabarnaamaa that the stone belonged to an unnamed king of Maalavaa in 1294. He estimated the value of the stone to be such as to feed the world for two days.
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Shaah Jahaan placed this stone in his Peacock Throne. When his son Aurangzeb had imprisoned him at nearby Aagaraa Fort, the legend has it that he had positioned the diamond near a window, so that he could see Taaj Mahal only looking at its reflection in the stone. It stayed there until Naadir Shaah until the invasion of Naadir Shaah on Dehlee and Aagaraa in 1739. It was he who seeing it, exclaimed "Kohinoor", otherwise nobody knew it by this name till then.

After the assassination of Naadir Shaah in 1747, the stone came into the hands of Ahmad Shaah Abdaalee of Afgaanistaan. In 1830, the deposed ruler of Afgaanistaan, managed to flee with Kohinoor to Laahaur, where it was given to Sikh Mahaaraajaa of Panjaab, Ranjeet Sinh. Ranjeet Sinh was able to persuade East India Company to lend him their troops and win back the Afgaanistaan throne for Shaah Shujaa (of Durraanee Empire of Afgaanistaan; Nov 4, 1785-Apr 5, 1842; ruled there during 1803-1809, and 1839-1842). It was Lord Dalhousie who procured this diamond from Ranjeet Sinh for British Empire.
[From Wikipedia, "Kohinoor"]