温 (Kanji for hot or warm thing)

Onyomi

The sun shining ON the plate full of water, made the water warm.

あたた*かい	also abbreviated to あったかい: warm thing (puppy, or puddle under your bed ) ★★★★☆
( を ) あたた＊める	I warm up (the muffins) ★★★☆☆
( が ) あたた＊まる	something gets warmed up ★★★☆☆

温度(おんど)

temperature ★★★☆☆
温 (hot or warm thing) + 度 (times) = 温度 (temperature)

温泉(おんせん)

hot spring bath ★★★☆☆
温 (hot or warm thing) + 泉 (spring) = 温泉 (hot spring bath)

hot spring baths (although most onsen use artificially heated water from regular wells these days)