A word meaning either “veil, screen, hiding” (Ety/SKAL¹; SA/esgal) or “a cast shadow” (PE17/184) derived from the root √SKAL “cover, veil”, most notably an element in the name Esgalduin (S/121) translated “River under Veil” (Ety/SKAL¹) or “River under Shade” (PE17/15, 184).

Conceptual Development: In The Etymologies of the 1930s esgal was translated “screen, hiding, roof of leaves”, but there it (and the river name) were from the Ilkorin language. In notes from around 1966-67, however, Tolkien shift the sense of the root, saying:

√SKAL was applied to more opaque things that cut off light and cast shadows over other things ... √SPAN was applied to things of lighter texture, and corresponds closer to our “veil” ... They appear also to have differed in that while SKAL was primitively verbal SPAN was primitively nominal. Thus the most primitive derivative of SKAL was skalā and this meant the action or effect of overshadowing: a cast shadow, S esgal, Q †ixal & hala. But spanā meant a thing that veiled, a veil (PE17/184).

Neo-Sindarin: For purposes of Neo-Sindarin, I think esgal can refer to both a shading screen (as it did in the 1930s) or the shadow under that screen (as it did in the 1960s), but it is unlikely that Tolkien himself maintained these two senses simultaneously. However, there are a number of other Sindarin good words for “shadow” but not many for “screen, veil”, especially since fân < ✶spanā or ✶phanā is used mainly with the sense “cloud”. However, I would limit esgal to genuinely opaque screens and curtains, ones that block most if not all light, and for a diaphanous veil I’d use fân.

A noun meaning “screen, hiding, roof of leaves” derived from the root ᴹ√SKAL¹ (Ety/SKAL¹). Apparently the initial [sk-] became syllabic [ṣk-], which then became [esk-]. Afterwards the [esk] became [esg].