A root Tolkien introduced in the late 1950s to serve as the basis for various “land” suffixes in Sindarin. The most detailed explanation appeared in a set of documents labeled “Changes affecting Silmarillion nomenclature”:
> The endings -ion, -ien, -ian(d) in place names. These have various origins. In Sindarin -ion is usually from -ı̯aun. This in origin is from yānā, √YANA-, extension of yā- (cf. YAGA, gap) “wide, large, extensive”. S iaun “roomy, wide, extensive” ... -iand (-ian) is from yandē “a wide region, or country” ... This was often used in plural of a single country (especially if it contained a varied topographical apsect) > iend, ien (PE17/42).
Thus all three suffixes -ian(d), -ien(d), -ion originate in the root √YAN. The suffix -ian(d) “land” (Beleriand) is the simplest, just a reduction of ancient -yandē. The suffix -ien(d) “lands” (Anórien) is a plural variant of -ian(d). The suffix -ion (Eregion) is S. iaun “wide, extension” used as a suffix (so perhaps = “✱extent”), becoming -ion because of the usual sound change whereby au become o in polysyllables.
In this same document, Tolkien also considered introducing a root √YŎNO “wide, extensive”, going so far as (temporarily) rejected the very well established word Q. yondo “son”. This √YON was blended with √YOD “fence, enclose”, and served as the basis for the suffix -ion, but Tolkien ultimately marked these notes with an “X” to reject them, perhaps because they only explained the suffix -ion, whereas √YAN could explain all three suffixes.
√YAN was mentioned in passing in other documents from this period, variously glossed “vast, huge” (PE17/99), “wide” (PE17/115), and “extend” (PE17/155), and in one place given a variant √YAD (PE17/115). In notes from December 1959 (D59) √YAN was contrasted with √ƷAN, the former meaning “wide” and the latter meaning “long”, both with the basic sense “extend” (PE17/115); see the entry on √HAN for further discussion.