Final answer:
The hard interior of a peach is known as a stone, which is a 5-letter word describing the hard shell that encloses the seed of a drupe fruit.
Step-by-step explanation:
The 5-letter word for the hard interior of a peach is called a stone. This term refers to the hard, inner layer that encloses the seed of a drupe, a type of fruit. Peaches, plums, and cherries are examples of drupe fruits that have a fleshy outer part surrounding a pit or stone that in turn encases the seed. The stone is composed of a hard shell that protects the seed until conditions are right for it to germinate and grow into a new plant.