before a vowel, dec-, word-forming element meaning "ten," from Latinized combining form of Greek deka "ten" (from PIE root *dekm- "ten"). In the metric system, "multiplied by ten;" while deci- means "divided by ten."
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dasa, Avestan dasa, Armenian tasn, Greek deka, Latin decem (source of Spanish diez, French dix), Old Church Slavonic deseti, Lithuanian dešimt, Old Irish deich, Breton dek, Welsh deg, Albanian djetu, Old English ten, Old High German zehan, Gothic taihun "ten."
decahedron n.
in geometry, "a solid having ten faces," 1828, from deca- "ten" + -hedron, from Greek hedra "seat, base, chair, face of a geometric solid," from PIE root *sed- (1) "to sit." Related: Decahedral.
Decalogue n.
"Ten Commandments," late 14c., from Latin decalogus, from Greek dekalogos, from the phrase hoi deka logoi used to translate "Ten Commandments" in Septuagint. See deca- + Logos.