| inherent(adjective, characteristic, essential, basic, fundamental) |
| involuntary(adjective, required) |
| life-or-death(adjective, important, essential, critical, basic) |
| necessity |
| paramount(adjective, important, significant, original, serious) |
| principal(adjective, important, significant, original, serious) |
| sine qua non(expression, idiom) |
| underlying(adjective, important, original, essential, basic) |
| basal(adjective, important, significant, original, essential) |
| bottom-line(adjective, important, significant, original, essential) |
| chief(adjective, important, significant, serious, capital) |
| constraining(adjective, required, obligatory) |
| elemental(adjective, important, significant, original, essential) |
| elementary(adjective, characteristic, important, significant, natural) |
| essence |
| essentials |
| exigent(adjective, important, significant, serious, essential) |
| inbred |
| ineluctable |
| intrinsic(adjective, characteristic, important, significant, natural) |