An adverb meaning 'each time' or 'whenever' — used to indicate that something happens on every occasion of a certain event.
日本を訪れるたびに、新しい店を見つける。
Every time I visit Japan, I find a new shop.
会うたびに彼は背が伸びている。
Every time I meet him he seems taller.
痛むたびに薬を飲んでください。
Take medicine whenever it hurts.