Henna is a small, flowering, tropical tree or shrub. Here's its scientific classification:

Kingdom: Plantae (Plants)

Division: Magnoliophyta (Flowering plants)

Class: Magnoliopsida (Dicotyledons)

Order: Sapindales

Family: Rutaceae (Citrus family)

Genus: Lawsonia

Species: Lawsonia inermis

The full scientific name is _Lawsonia inermis_, though it is often referred to as Henna.