A container for carrying water or drinks (canteen, flask, water bottle, thermos).
ハイキングには水筒を忘れないでください。
Don't forget your water bottle for the hike.
子供は遠足にお気に入りの水筒を持って行った。
The child took their favorite water bottle on the field trip.
暖かいお茶を水筒に入れて出かけた。
I put hot tea in a thermos and went out.