What do you call the core of a fruit?

Anatomically, many fruits consist of two primary layers: the pericarp and the seed. The pericarp is tissue that derives directly from ovary of a flower. It consists of three sub-layers:

• Epicarp: the outermost layer of a fruit; forms a tougher "skin" around the fruit
• Mesocarp: the middle layer of a fruit, found between the epicarp and the endocarp; usually (but not always) the fleshy part that is consumed
• Endocarp: the inner layer of a fruit that directly surrounds the seeds

Additionally, accessory fruits have fleshy tissue not derived from the ovary but from other parts of a flower.

Source: Wikipedia, "Fruit Anatomy"

Depending on the type of fruit, different layers may be physically "fleshy" and eaten. Here are some general guidelines:

• In pomes, such as apples, the majority of the flesh is not part of the pericarp (it is not derived from the ovary) but rather is "accessory tissue." The entire pericarp forms the core. (So, the epicarp is not the skin, as one might expect; it is encased inside the accessory tissue.)
• In drupes, such as peaches, the majority of the flesh is the mesocarp. The epicarp forms the skin. The endocarp forms the pit, which surrounds the seed.
• Berries vary depending on whether they are botanical berries or not, and by what type of berries they are. For example, blueberries and grapes (botanical berries) are eaten in their entirety.
• In contrast, strawberries are not botanical berries; they are accessory fruits like apples, so the flesh is accessory tissue. Each "seed" on the strawberry is called an achene, which is like a miniature fruit including both pericarp and a single seed that fills most of the volume.
• In hesperidium (a modified berry), which are citrus, the epicarp forms the peel, the mesocarp forms the pith, and the endocarp forms the juicy flesh that is eaten.
• From what I can find on bananas (also botanical berries), the epicarp forms the peel, while the mesocarp and endocarp form the flesh that is eaten.

There are several terms used, depending on the size, number and where you live.

If there are lots of small ones (grapes, apples, oranges): if you are American, you would call them seeds and, if you are British, you would call small ones seeds and slightly bigger ones (especially citrus fruit) pips.

If there is just one large one (olives, cherries, peaches etc): if you are American, you would call it a pit and, if you are British, you would call it a stone.

You can use this NGRAM graph to experiment with British/American terms for different fruits. As the NGram graph shows, these are not hard and fast rules.

One notable exception is processed fruits like dates and olives. Olives are mainly grown, prepared and packed in non-English speaking countries. They supply to both US (141,000 tonnes per year) and UK (1,600 tonnes per year). It's easy to see from these figures why the suppliers choose to use the American term "Pitted Olives" on their packaging, even on products supplied to the UK. The import figures were obtained from here and here.

The word fruit matured in Middle English and grew from the seeds of Anglo-French frut and fruit, which are rooted in the Latin verb frui, meaning "to enjoy" or "to have the use of." Scientifically, it is the name for the fleshy or dry ripened ovary of a flowering plant enclosing a seed or seeds. So, apricots, bananas, grapes, as well as bean pods, corn grains, tomatoes, cucumbers, and (when in their shell) acorns and almonds are all fruits—fruits are not necessarily sweet, succulent, or pulpy foods. The botanical use of ovary has similarities in meaning to the one found in human anatomy textbooks.

In botany, ovary applies to the part of a flower where seeds are formed. The ovary bears ovules, which enclose the seeds, and consists of one or more carpels. Ovary derives from Latin ovum, meaning "egg," and ovule is the Latin diminutive. Carpel also is seeded in Latin.

Carpel derives from New Latin carpellum, and, in turn, Greek karpos, meaning "fruit." A homograph of karpos means "wrist," which gives English carpal, as in carpal tunnel syndrome, as well as carpus, which is used in medical writing as a name for the joint. The fruity homograph refers to the ovule-bearing structures in an angiosperm that comprises the innermost whorl of a flower that matures into a fruit. From the Greek root the science terms exocarp, mesocarp, and endocarp sprouted. Together, they designate the regions of what comprises the fruit's pericarp.

The prefix peri- is from Latin and Greek and indicates something that is all around, enclosing, or surrounding. The pericarp is considered the all-encompassing wall, or walls, of the fruit. Citrus fruits, such as the grapefruit, also have membranous walls within the pericarp that separate the fruit into segments containing the pulp and seeds—they aren't very tasty but they have a purpose. On the other hand, you will find the combined juice sacs in the segments that are divided by the membranes and that make up the fruit's pulp tasteful.

Exocarp (a.k.a., epicarp) refers to the outermost layer of the pericarp—which is variously called the skin, rind, peel, or husk of a fruit—that covers the mesocarp. Mesocarp refers to the middle layer of a fruit, the usually fleshy plump part (in grapes, it contains funiculi, strands that connect the seed to the grape's pedicel). In some fruits, such as citrus fruits, the mesocarp includes the albedo, which you may know as the inner whitish, spongy, tasteless layer of the rind. (In Latin, albus means white.)

Endocarp is the name for the inner region of the pericarp and can consist of layers of different textures or consistency that surround and protect the seed. It varies in structure—compare, for example, the core, stone, or pit of a peach, apple, cherry, olive, mango, walnut, or orange. Each is unique in some way, shape, or form.

At the center of fruit is the seed or pip encapsulated by the endocarp. The word seed was planted in Old English. Pip, as the name for a small fruit seed (the word also has various other meanings in English), was planted in the 18th century. This pip was cultivated from Anglo-French pepin, and the English variation, pippin, is the name of a crisp, tart apple (especially good for pies and the like), as well as a person or thing that is, or should be, highly admired.

"I wish, darling, you could have seen my drive at the eighth just now. It was a pip!"
— P. G. Wodehouse, The Clicking of Cuthbert, 1922

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