Molech: the name attached to the rite in which children were burned in the Valley of Hinnom, understood either as the god the offering was made to or as the offering itself
Every time this word appears in the Hebrew Bible it stands in the same dark setting: a rite, carried out at a place called the Tophet in the Valley of Hinnom just outside Jerusalem, in which children were put to death by fire. The Bible treats the rite as the worst thing a person in Israel could do, and it ties the word Molech directly to it.
What the word actually names is the great unsettled question. On one reading it is the name of a foreign god, a king-like lord of the dead to whom the children were offered, which is how the texts that speak of "whoring after" Molech and of a high place built "for" Molech most naturally read. On the other reading it is not a god at all but the technical name of the sacrifice itself, a kind of offering, so that to "give your child to the molech" means to give the child as a molech-offering. The same handful of verses can be read either way, and scholars remain split (see the dispute below).
Whatever the word first meant, the Bible's writers use it to push the practice as far from the worship of Israel's God as they can. The standard vowels of the name appear to have been bent to make it sound like the Hebrew word for "shame," turning what may once have read "the King" into something like "the Shame-king." The Valley of Hinnom where the rite took place later gave its name, through Greek, to the word the New Testament uses for "hell."
Because the word names something both foreign and abhorrent, English Bibles do not translate it. They carry it over as a name, Molech or Moloch, and let the surrounding verses explain what it was.