The word according to the dictionary

Ancient Greek

From Proto-Indo-European *dʰh₁-m-eló-, from *dʰeh₁- (“to put”).

θεμέλῐος • (themélios) m or f (neuter θεμέλῐον); second declension

belonging to the foundation
(as a masculine or neuter substantive, sc. λίθος (líthos)) foundation-stone, foundation, basis

Number	Singular		Dual		Plural
Case/Gender	Masculine / Feminine	Neuter	Masculine / Feminine	Neuter	Masculine / Feminine	Neuter
Nominative	θεμέλῐος themélios	θεμέλῐον themélion	θεμελῐ́ω themelíō	θεμελῐ́ω themelíō	θεμέλῐοι themélioi	θεμέλῐᾰ themélia
Genitive	θεμελῐ́ου themelíou	θεμελῐ́ου themelíou	θεμελῐ́οιν themelíoin	θεμελῐ́οιν themelíoin	θεμελῐ́ων themelíōn	θεμελῐ́ων themelíōn
Dative	θεμελῐ́ῳ themelíōi	θεμελῐ́ῳ themelíōi	θεμελῐ́οιν themelíoin	θεμελῐ́οιν themelíoin	θεμελῐ́οις themelíois	θεμελῐ́οις themelíois
Accusative	θεμέλῐον themélion	θεμέλῐον themélion	θεμελῐ́ω themelíō	θεμελῐ́ω themelíō	θεμελῐ́ους themelíous	θεμέλῐᾰ themélia
Vocative	θεμέλῐε themélie	θεμέλῐον themélion	θεμελῐ́ω themelíō	θεμελῐ́ω themelíō	θεμέλῐοι themélioi	θεμέλῐᾰ themélia
Derived forms	Adverb		Comparative		Superlative
Derived forms	θεμελῐ́ως themelíōs		θεμελῐώτερος themeliṓteros		θεμελῐώτᾰτος themeliṓtatos
Notes:	This table gives Attic inflectional endings. For declension in other dialects, see Appendix:Ancient Greek dialectal declension.

“θεμέλιος”, in Liddell & Scott (1940) A Greek–English Lexicon, Oxford: Clarendon Press
“θεμέλιος”, in Liddell & Scott (1889) An Intermediate Greek–English Lexicon, New York: Harper & Brothers
θεμέλιος in Bailly, Anatole (1935) Le Grand Bailly: Dictionnaire grec-français, Paris: Hachette
Bauer, Walter et al. (2001) A Greek–English Lexicon of the New Testament and Other Early Christian Literature, Third edition, Chicago: University of Chicago Press
G2310 in Strong, James (1979) Strong’s Exhaustive Concordance to the Bible
Woodhouse, S. C. (1910) English–Greek Dictionary: A Vocabulary of the Attic Language‎, London: Routledge & Kegan Paul Limited.
- foot idem, page 333.