The Collaborative International Dictionary
Helio-
Helio- \He"li*o-\ (h[=e]"l[i^]*[-o]-). A combining form from Gr. "h`lios the sun.
Douglas Harper's Etymology Dictionary
helio-
word-forming element meaning "sun," from Greek helio-, comb. form of helios "sun" (see sol).
Wiktionary
helio-
pre. Concerning the sun.