The Areca nut is the seed of the Areca

Areca

Areca is a genus of about 50 species of single-stemmed palms in the family Arecaceae, found in humid tropical forests from Malaysia to the Solomon Islands. The generic name Areca is derived from a name used locally on the Malabar Coast of India....

 palm (Areca catechu

Areca catechu

Areca catechu is the areca palm or areca nut palm, , a species of palm which grows in much of the tropical Pacific, Asia, and parts of east Africa. The palm is believed to have originated in either Malaysia or the Philippines...

), which grows in much of the tropical Pacific, Asia, and parts of east Africa. It is commonly referred to as "betel nut" as it is often chewed wrapped in betel

Betel

The Betel is the leaf of a vine belonging to the Piperaceae family, which includes pepper and Kava. It is valued both as a mild stimulant and for its medicinal properties...

 leaves.
The areca nut is not a true nut but rather a drupe

Drupe

In botany, a drupe is a fruit in which an outer fleshy part surrounds a shell of hardened endocarp with a seed inside. These fruits develop from a single carpel, and mostly from flowers with superior ovaries...

. It is commercially available in dried, cured and fresh forms. While fresh, the husk is green and the nut inside is so soft that it can easily be cut with an average knife.