The Etymologies is a chapter of The Lost Road and Other Writings, the fifth volume of The History of Middle-earth. In his preface to that volume, Christopher Tolkien describes The Etymologies as "a kind of historical dictionary", and associates the work with the years 1937-1938, prior to his father's work on The Lord of the Rings.
In The Etymologies, Tolkien provides a series of Elvish linguistic root syllables, or "bases". He then shows how each base evolved into various words in the various Elvish languages, primarily Quenya and Sindarin (then called "Noldorin"), but also about ten other variations. Although the derived words cannot necessarily be said to be in their final forms, as Tolkien's invented languages continued to evolve throughout his life, as Christopher Tolkien points out, this document "enormously increases the known vocabularies of the Elvish tongues".
Addenda and Corrigenda
In 2003 and 2004, Vinyar Tengwar issues 45 and 46 provided addenda and corrigenda to the original published text of The Etymologies, with previously unknown entries and roots. In 2007, Parma Eldalamberon issue 17 provided even more entries.