anything indispensable
of the greatest importance
absolutely necessary; vitally necessary
defining rights and duties as opposed to giving the rules by which rights and duties are established
basic and fundamental
being or relating to or containing the essence of a plant etc
far-reaching and thoroughgoing in effect especially on the nature of something
being or involving basic facts or principles
the lowest tone of a harmonic series
serving as an essential component
any factor that could be considered important to the understanding of a particular business