As substitute refers to an alternative or replacement for something or someone. The antonyms for this phrase could include original, primary, necessary, essential, or indispensable. These antonyms convey the idea that the item or individual referred to as a substitute is not the preferred or ideal choice, but rather an option in the absence of the original or necessary entity. For example, a substitute teacher is not the primary instructor, and their presence is temporary until the original teacher returns. Similarly, a substitute ingredient in a recipe may not be the essential component, and therefore the dish may lack its intended flavor or texture when the original ingredient is omitted.