[es.sen.tial] adj (14c) 1: of, relating to, or constituting essence: inherent
2: of the utmost importance: basic, indispensable, necessary "~ foods" "an ~ requirement for admission to college"
3: idiopathic "~ disease" -- es.sen.tial.ly adv -- es.sen.tial.ness n syn essential, fundamental, vital, cardinal mean so important as to be indispensable. essential implies belonging to the very nature of a thing and therefore being incapable of removal without destroying the thing itself or its character "conflict is essential in drama". fundamental applies to something that is a foundation without which an entire system or complex whole would collapse "fundamental principles of algebra". vital suggests something that is necessary to a thing's continued existence or operation "cut off from vital supplies". cardinal suggests something on which an outcome turns or depends "a cardinal rule in buying a home".
[2]essential n (15c) 1: something basic "the ~s of astronomy"
2: something necessary, indispensable, or unavoidable