I am having trouble finding the right word. What do you call the hard, uneatable parts inside a fruit? For example, an avocado has one big dark one. Grapes have small ones. Watermelons have a lot of black ones.
I looked it up in the dictionary and it gives "seeds", but I wonder if that is in common use in everyday language.
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.
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.
As another suggestion, most generally, you can refer to the central "inedible" part of a fruit as the core:
the hard central part of some fruits, such as apples, that contains the seeds
Example: a pineapple core
The process of removing the core is called coring:
remove the tough central part and seeds from (a fruit)
Example: Peel and core the pears before cooking them.
Here's a picture of a person using an apple coring device called fruit corer to core an apple:
The large hard thing in an avocado is called a pit. The small ones in grapes and watermelons are called seeds.
a large, hard seed that grows inside some types of fruit and vegetables: a peach/plum/cherry pit
a small, usually hard part of a plant from which a new plant can grow
I found this site (differencebetween.net) that talks about the difference. Here is an excerpt:
Seeds vs Pits
A seed is an ovule that contains an embryo inside, enclosed in a seed coat and usually contains some food. A seed results after fertilization when the ovule ripens. When the seed forms in seed plants, it marks the completion of the reproduction process. Reproduction begins with flower development and pollination, with the embryo developing from the zygote and the seed coat from the ovule’s skin.
The pit is the part of the fruit that protects the seed until such time when it can start to grow. It is the inner layer of a fruit’s (some fruits) pericarp that’s usually hard. However, only certain fruits have a pit. While there can only be one pit in a fruit, many seeds can be contained in a single fruit and this is a key differentiating factor of a seed and a pit, for instance a cherry contains a pit while a grape has got seeds. Other fruits with pits include olive, dates and plums.
It is hard to make definitive categories, but broadly in UK, inside a fruit:
Seeds are usually small and edible. Examples are kiwi fruit, fig and strawberry but grape seeds are inedible. Pumpkin and melon seeds are larger, and edible.
Pips are usually larger and inedible. Examples are orange, grapefruit and apple but tomato and pomegranate pips are edible.
Stones are larger and inedible. Examples are peach, apricot, date and cherry.
Pits are stones that are labelled on imported produce, such as dates and olives.
There are some borderline examples such as the soft fruits blackberry and raspberry whose pips can be eaten raw but stick in the teeth. They can be softened by cooking in a pie.