The Avari, also known as the Silvan or Wood-Elves, are a race of Elf to have split along with the Eldar.
At the Time of Awakening, all Elves lived in the East of Middle-earth near Orocarni, the Mountains of the East, on the shore of Helcar, the Inland Sea. But in time the summons of the Valar came and all the Elves had to make a choice between starlight and the promise of a land of eternal light. Those who chose eternal light and set out on the Great Journey were named the Eldar, while those who remained were called the Avari, the "unwilling".
The Avari became a less powerful people, for their land became barbarous with Dark Powers and evil races, and so the Elves dwindled and hid themselves. They became as shadows and sprites that mortal eyes could not perceive. They lived always close to the wooded land, built no cities and had no kings. Later, in the years of Sun, the Avari were named Silvan or Wood-elves, and some of them wandered westwards and became involved in the great affairs of their Eldar kindred under whom they grew prosperous and strong for a while, before dwindling once again.