The “nut” in “betel nut” is actually the seed of the areca palm (Areca catechu).

The palm produces a fruit similar in general appearance to a small palm fruit, within which is the hard, fibrous seed.

The seed is what is used; the fruit flesh is not.

In botanical terms, the seed is not a nut at all.

The “betel” in “betel nut” refers not to the palm but to the betel vine (Piper betle), a completely different plant.

The betel vine’s leaves are what the nut is traditionally wrapped in when making the betel quid.