tellus

Tellus, telluris is third declension and feminine, despite the -us that makes every learner reach for a second declension masculine. Genitive telluris, ablative tellure. It has effectively no plural.

It is a poetic and elevated word where terra is the everyday one, so expect it in verse, in solemn prose and in geographical writing that fancies itself. Capitalised, Tellus is the earth goddess, the Roman counterpart of Greek Gaia, with her own temple on the Esquiline. The chemical element tellurium is named from it, and English kept telluric for anything belonging to the earth; but for ordinary purposes, when you want to say "land" or "country", use terra and keep tellus for effect.