Indispensible

1. vital

2. primary

3. Central

4. Key

5. crucial

5.1. Crux

6. essential

6.1. critical

6.2. basic

6.3. foundation

6.4. groundwork

6.5. heart

6.5.1. core

6.5.1.1. connected

6.5.1.2. linked

6.5.1.3. coherent

6.5.2. center

6.5.3. focus

6.5.4. origin

6.5.5. energy

6.6. bottom line

7. fundamental