essential

If something is essential, it is impossible to succeed without it.

Example: Oxygen is essential for life on earth.

If something is essential, it is very important.

Example: Cooked rice, is the essential element of the Chinese meal prepared south of the Yangtze River.

The essential part, idea, or feature is the most basic one.

Example: The essential nature of skiing is sliding on two skis, but the sport is much more than that.