かんじん肝心kanjin
1.

Pertaining to something that is of utmost importance or fundamental, often regarded as key to understanding or achieving a goal.

2.

Describing a crucial element or aspect that cannot be overlooked, which serves as the foundation for a broader context or set of actions.

3.

Indicative of that which is vital for success or operation, emphasizing its irreplaceable nature in relation to a specific task or situation.

Example:

なんであれ物事の本体を捕らえる事が肝心だ。

かんじんかなめ肝心要kanjinkaname
1.

Indispensable; referring to something that is fundamentally important or essential in a given context.

2.

The crucial element or aspect that significantly affects the outcome or essence of a situation.

3.

The central focus or key aspect that cannot be overlooked or ignored.

Example:

そう言えば、肝心要の部分を何も決めていなかった。