NTC thermistor
A resistor whose resistance falls steeply as temperature rises (Negative Temperature Coefficient) — typically halving every ~20 °C. Paired with a fixed resistor in a divider, it turns temperature into a voltage.
In practice
The β equation converts resistance to temperature; the divider's fixed resistor should equal the NTC's resistance at your most-important temperature for best sensitivity there. Big power NTCs do double duty as inrush limiters.