肝心

Etymology
From +  or.

.

Adjective

 * 1)  vital, essential, main

Noun

 * 1)  that which is essential or fundamental

Usage notes
Prior to the addition of the character to the Jōyō Kanji List in 2010, that character was substituted with, producing 肝心. The new standard recommends instead, but 肝心 is still more widely used.

Etymology
From +.

Noun

 * 1) thoughtfulness, sincerity