The Moriquendi (Q. ‘Dark-Elves’) were those Elves who never undertook the Great Journey over the Sea and who therefore never beheld the light of the Two Trees of Valinor. Counted among the Moriquendi are two branches of the Teleri – the Nandor and Sindar* Elves – and all of the Avari (‘Unwilling’). Those who did initially begin the journey but did not finish were known as the Úmanyar (‘not of Aman’).
*It should be noted that while Thingol was a Sindarin King, he had traveled to Valinor prior to returning to Middle-earth; therefore, he is not counted among the Moriquendi.