Is a coconut's edible part the endocarp?

The part of the coconut that is usually eaten (as coconut "meat" and water) is the endosperm.

This excerpt from the entry for coconuts (in Wayne's World online textbook of natural history, from Palomar College) gives an explanation of the "multinucleate liquid endosperm" in a coconut:

"The fruit of the coconut (Cocos nucifera) is technically a large, dry drupe (D) composed of a thin outer layer (exocarp), a thick, fibrous middle layer called a mesocarp (F), and a hard inner layer called an endocarp (E) that surrounds a large seed. The endocarp (A) contains three germination pores at one end, one of which the sprouting coconut palm grows through.

The "meat" of the seed is endosperm tissue (B) and a small, cylindrical embryo is embedded in this nutritive tissue just opposite the functional germination pore. The seed is surrounded by an outer brown layer called the seed coat or testa. This is the brown material that adheres to the white "meat" or endosperm when it is removed from the endocarp shell. "Coconut water" (C) is multinucleate liquid endosperm that has not developed into solid tissue composed of cells. Copra comes from the meat of dried coconuts, while coir fibers are derived from the fibrous mesocarp."

More on the endosperm, from Handbook of the Coconut 2019, TetraPak

Coconut water is the liquid endosperm found in the cavity of the nut. By the third month of fruit development, there are small quantities of coconut water. This amount increases and reaches the maximum when the nut is 7-9 months old. This is also when the coconut water tastes the sweetest, and is classified as young coconut water...Coconut water harvested from nuts between 10-13 months old is classified as mature coconut water. After the nuts ripen, the amount of coconut water declines. This is because during maturation, coconut water is used to form coconut flesh inside the fruit, a phenomenon across all of the coconut’s varieties.

More information on coconuts: