1. Something that is basic or necessary for the present and in the future: "The teacher made every effort to teach her students the essentials of vocabulary words; including, their etymologies and practical usages."
2. That which is needed for life, or needed in particular situations: "There are certain essentials that a person needs to know for success in this business."

Essential and Necessary are used interchangeably

The two terms refer to that which is indispensable or required for the completion or the accomplishment of something.

• Water is essential (or necessary) to sustain life of creatures both the greatest and the smallest.
• It is necessary (or essential) that the majority of citizens vote if they want to have a "government of the people, by the people, and for the people".

Essential is the stronger word because it means "pertaining to the very essence of existence". Necessary expresses a strong desire and can be some form of compulsion; however, essential implies that if something is not done, it can no longer exist.