| inbred(adjective, basic, absolute, fundamental) |
| indispensable(adjective, important, significant, original, essential) |
| inherent(adjective, essential, basic, absolute, fundamental) |
| innate(adjective, basic, absolute, fundamental) |
| main(adjective, original, essential, basic, absolute) |
| material(adjective, basic, absolute, fundamental) |
| meat and potatoes(expression, adjective, idiom, simple, plain) |
| name of the game(expression, adjective, idiom, basic, absolute) |
| necessary(adjective, important, significant, original, essential) |
| nitty-gritty(adjective, original, basic, absolute, fundamental) |
| nub(adjective, original, basic, absolute, fundamental) |
| perfect(adjective, basic, absolute, fundamental) |
| prime(adjective, important, significant, original, essential) |
| principal(adjective, important, significant, original, essential) |
| radical(adjective, important, significant, original, essential) |
| vital(adjective, important, significant, original, essential) |
| absolute(adjective, basic, fundamental, central) |
| capital(adjective, essential, fundamental, principal) |
| chief(adjective, essential, fundamental, principal) |
| critical(adjective, original, essential, basic, fundamental) |