the skull: the bony head, and so a single head as the unit for counting people one by one.
This word names the hard, round bone of the head. It turns up in some of the bleakest scenes in the Hebrew Bible: a millstone dropped from a tower crushes Abimelech's skull, Saul's skull is nailed up in a Philistine temple, and of Jezebel nothing is found but the skull, the feet, and the palms of the hands.
From there it becomes a counting word. Hebrew tallies people the way English still does, one head at a time. The lists in Numbers count every man "by their heads," and a fixed payment is set at so much "per head." Nothing grim is meant by it. The head is simply the visible, countable unit of one person, exactly as English says "a head count" or "ten dollars a head."
Hebrew's ordinary word for the head is רֹאשׁ, which also stretches to mean the top of a thing, the beginning of a stretch of time, or the chief of a group. This word never does any of that, and its narrowness is what makes it useful for counting: a "head" in this sense can never be mistaken for a leader. Judges 9:53 sets the two side by side, the millstone landing on Abimelech's head and shattering his skull, and 1 Chronicles 23:24 does the same, naming the heads of the families and then counting the Levites one head at a time.