noun
ice pack; cold pack; thermal gel pack
A reusable gel pack used in coolers or lunch boxes to keep food cold. Often called an 'ice pack' in everyday English, though it doesn't contain ice.
お弁当に保冷剤を入れてください。
Please put an ice pack in the lunch box.
クーラーボックスに保冷剤を入れておいた。
I put some cold packs in the cooler.
冷却剤 is a more general term for any coolant or refrigerant, often used in industrial or chemical contexts, while 保冷剤 specifically refers to the consumer gel packs for keeping things cold.
Compound of 保冷 (keep cold) + 剤 (agent, preparation). A straightforward modern coinage for a common household item.