STEL is a root signifying "remain firm". A derivative of STEL is said to be contained as the "[l]atter element" in the names Ecthelion and Aegthelion.
J.R.R. Tolkien, Christopher Tolkien (ed.), The War of the Jewels, "Part Three. The Wanderings of Húrin and Other Writings not forming part of the Quenta Silmarillion: III. Maeglin", p. 318 (form "√skel"