kimariji; identifying characters of a poem
In competitive karuta, the minimum initial characters of a poem that uniquely identify it, used to determine which card to take.
See also: 百人一首
Memorizing the kimariji of the Hyakunin Isshu is tough.
決まり字が読まれた瞬間に札を取る。
You take the card the moment the kimariji is read.
Compound of 決まり (kimari, 'decided, fixed') and 字 (ji, 'character'). The term is specific to karuta and refers to the fixed set of initial characters that uniquely identify a poem.