Sindarinization was the process in which various Quenya names and words were changed to fit Sindarin rules and style. Such names included the major changes of Noldofinwë to Fingolfin and Nelyafinwë to Maedhros. Less striking differences included Angaráto to Angrod and Itarillë to Idril.
History of Sindarinization
After the Flight of the Noldor, the Noldor and the Sindar began to find a medium of communication with one another. In learning one another's languages, the Noldor caught quickly onto Sindarin, while the Sindar had extreme difficulty with Noldorin (Quenya). This was the first step towards Sindarinization.
The next step came quickly and absolutely. Elu Thingol learned of the Kinslaying at Alqualondë, and was horrified and angered at the massacre of his kin. He declared that the tongue of the kinslayers would not be spoken in his hearing, and commanded all Sindar not to speak it or answer to it, and to shun those who spoke it. Therefor the Noldor were forced to speak Sindarin, and it became their tongue in daily use. But Quenya was not blotted out completely from the Elves of Beleriand, and it stayed still in their lorebooks, and was spoken in private among the lords of their kindred.