A word for “Earth” used in the Sindarin translation of the Lord’s Prayer from the 1950s, in the phrase: bo Ceven sui vi Menel “on Earth as [it is] in Heaven” (VT44/21). It is clearly a cognate of Q. cemen of the same meaning, and like Quenya I suspect this word can be used for both “Earth” and “earth”. The more usual Sindarin word for “world” was amar, so I suspect that, where referring to the global realm, ceven meant more the “habitable surface of the earth” rather than the entire planet. See, for example Christopher Tolkien’s note on kemen “referring to the earth as a flat floor beneath menel, the heavens” from The Silmarillion appendix (SA/kemen).
Possible Etymology: If this word is indeed a direct cognate of Q. cemen < ✱kemen, it is not clear why the final n didn’t vanish as was usual in Sindarin; perhaps the Sindarin form was derived from a variant primitive form like kemenē. Alternately, it may be a back-formation from some inflected form, as happened with other similar words like S. aran and S. thoron.