noun
mouthful; bite; morsel
Used for a small amount of food taken into the mouth at once, or for taking just one bite.
このリンゴを一口食べてみて。
Try a bite of this apple.
Used for a small amount of liquid drunk at once; depending on context it can be a sip or a gulp.
彼女は水を一口飲んでから話し始めた。
She took a sip of water and then began to speak.
noun
in a word; in short; one brief word
Common in set expressions such as 一口で言えば or 一口に言えば, meaning to sum something up briefly.
一口で言えば、計画は失敗だった。
In short, the plan was a failure.
noun
one share; one unit of contribution; one subscription unit
Used as a unit for investments, subscriptions, donations, and similar contributions; often followed by an amount of money.
寄付は一口千円から受け付けています。
Donations are accepted from 1,000 yen per contribution unit.
この投資信託は一口一万円で購入できる。
This investment trust can be purchased in units of 10,000 yen.