1. Being of the greatest importance of something; inherent: "They decided to pack only the essential items for their camping trip."
2. Basic or indispensable; necessary: essential ingredients: "The cook considered garlic to be an essential component of his soup."

"Free speech is supposed to be an essential right of U.S. citizens."

3. In biochemistry, a substance that is required for normal functioning but cannot be synthesized by the body and therefore it must be included in the diet: "The doctor told him that he had to include essential amino acids in his food consumption."