A word for “radiance, glittering reflection” derived from primitive ✶ñ(g)alatā and the root √Ñ(G)AL “shine by reflection” (PM/347). Most notably it was an element in the names Galadriel and Gil-galad. In the latter name, since the ancient form began with ñg-, its mutated form was -galad rather than the more usual ’alad.

Conceptual Development: This word had a long and intricate history, and was intertwined with the shifting conceptions of various roots and names. It’s earliest iteration was G. {gâl(a) >>} gala “light, daylight” (GL/37), cognate to ᴱQ. kala of similar meaning, derived from the early root ᴱ√KALA “shine golden” (QL/44). Since ancient initial g- became k- in Early Qenya, is very likely the early root was actually *ᴱ√GALA. In early notes, Tolkien experimented with various alternate forms for this word such as gâl or gaul (GL/37; PE13/114), and in Early Noldorin Word-lists of the 1920s Tolkien had the word ᴱN. galad with the gloss “dawn” (PE13/144).

By the 1930s, Tolkien had revised the phonology of Quenya so that primitive initial g became ʒ and then vanished, but he wished to retain the kal-/gal- variation in “light” words, and so introduced a Noldorin-only variant ᴹ√GAL of the root ᴹ√KAL “shine” (Ety/GAL). In The Etymologies, Tolkien had both N. calad (Ety/KAL) and N. galad (EtyAC/GAL) for “light”, the former appearing in its mutated form as an element in the name N. Gil-galad “Starlight” (Ety/GIL).

S. calad continued to be the basis for the second element of S. Gil-galad in some later writings (PE17/50), but Tolkien began to have problems with another name from The Lord of the Rings, that is Galadriel. When Tolkien first introduced her name, it basically meant “Tree-lady” (TI/249), but this was no longer suitable for her more elevated role in the Legendarium, and in any case the proper Sindarin word for “tree” was galadh. In 1955 notes Tolkien considered making the initial element of her name mean “blessed” or “bliss” instead (NM/346).

In notes from the late 1950s or early 1960s Tolkien connected the names Gil-galad and Galadriel as sharing a common element meaning “light” and experimented with various derivatives of the roots √KAL and √GAL (PE17/50), but couldn’t contrive a satisfactory solution since the shared word would be mutated in Gil-Galad but unmutated in Galadriel. Eventually Tolkien resolved this quandary by introducing a new root √Ñ(G)AL in various notes from the late 1960s (PE17/59-60; NM/353) whose primitive form in Sindarin was strengthened ✶ñgalatā that would produce the correct result for both names. Tolkien discussed this new word at length in The Shibboleth of Fëanor from 1968:

The name [Galadriel] was derived from the Common Eldarin stem ÑAL “shine by reflection”; *ñalatā “radiance, glittering reflection” (from jewels, glass or polished metals, or water) > Quenya ñalta, Telerin alata, Sindarin galad ... The whole = “maiden crowned with a garland of bright radiance” was given in reference to Galadriel’s hair. Galad occurs also in the epesse of Ereinion (“scion of kings”) by which he was chiefly remembered in legend, Gil-galad “star of radiance” (PM/347).

Neo-Sindarin: For purposes of Neo-Sindarin, I would assume (ñ)galad refers to radiant, glittering and reflected light, as opposed to S. calad (lit. = “shining”) which refers to direct light and the sources of such light, such as lamps and flames.