IPA: /ˌdʒiːəʊˈθɜːrməli/
KK: /dʒiˈoʊθɜrməli/
In a way that relates to or utilizes the heat from the Earth.
The power plant generates electricity geothermally, using heat from deep underground.
Geothermally → It is formed from "geo-" (from Greek "gē", meaning earth) and "thermal" (from Greek "thermos", meaning heat) with the suffix "-ly" (meaning in a manner). The word "geothermally" refers to something that occurs in a manner related to the heat of the earth.
Think of 'earth' ('geo-') and 'heat' ('thermal') to remember that 'geothermally' means in a way that relates to the heat from the earth.