A small bottle with an opening filled with liquid material to dose a yukkuri.
One of the most popular ampoules is sex pastes ampoule, which is filled with sex pastes for the purposes of impregnating a yukkuri without the need for refreshing.
In some older works where ampoules are mostly used to impregnate pet_yukkuris without the need of purchasing a husband, impregnation by an ampoule will result in the death of the impregnated yukkuri.
In more recent works, ampoules mostly shift to be used for livestock yukkuris instead and these "ampoule causes death" mechanic seems to mostly be forgotten in favor of long-term abuse.
There are also other types of ampoules, such as the orange juice ampoule meant for keeping yukkuris alive (see post #32417, post #67855).