Fennel n. : A perennial plant of the genus Faeniculum (F. vulgare), having very finely divided leaves. It is cultivated in gardens for the agreeable aromatic flavor of its seeds.

Example:
How much Fennel leaf instead of Fennel seed?

Taxonomy
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Defination from the web:
Fennel (Foeniculum vulgare) is a plant species in the genus Foeniculum (treated as the sole species in the genus by most botanists) ... In Ancient Greek, fennel was called marathon (μάραθον), and is attested in Linear B tablets as ma-ra-tu-wo. ...