How to say "flask" in Japanese

• 水筒すいとうJLPT N2approx.Common
  nounCovers the everyday sense of 'flask': a portable container for carrying water, tea, or other drinks. This is the word you want for a canteen or thermos-style bottle taken to school, work, or outdoors.
  水筒すいとうにはほとんど水みずが残のこっていない。↗

  There is little water left in the canteen.
• 軍持ぐんじ
  nounRefers to a water bottle or flask carried by a Buddhist monk or nun, so it belongs to religious rather than everyday contexts. It is also a less common word, and seldom the one you want for an ordinary flask.
• トラベルマグ
  nounA loanword for a travel mug, matched to 'flask' in that portable-drinking sense. It is less common and fits mainly contexts involving mugs designed for carrying drinks while traveling, not flasks generally.

For the everyday drinking vessel, 水筒 is the default; 軍持 is a Buddhist monk's water flask, so it suits religious contexts; 「トラベルマグ」 is the borrowed term for a portable travel mug.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

• 水筒すいとうcanteen
• 軍持ぐんじwater bottle (of a monk, nun, etc.)
• トラベルマグtravel mug
• フラスコ(laboratory) flask