Trademarked brand name often used generically for vacuum-insulated flasks and bottles, similar to 魔法瓶 (まほうびん).
See also: 魔法瓶 (まほうびん)
On cold days, I put hot tea in a thermos and take it with me.
このサーモスは保温性がとても高い。
This thermos has excellent heat retention.