This is the 888th most frequent Japanese word.
冷
"Cold" or "cool."
冷たい (tsumetai) is used as an adjective meaning "cold" to describe the temperature of the water.
冷たい水を飲むと、気持ちがすっきりします。
Drinking cold water feels refreshing.
冷やす (hiyasu) is a verb meaning "to cool" or "to chill", demonstrating an action related to lowering temperature.
この道具を冷却機で冷やしてください。
Please cool this tool using a cooling device.
冷静 (reisei) incorporates "冷" to mean "calm" or "composed", reflecting a metaphorical use of "coolness".