What is "exacum"

Exacum is a genus of plant in family Gentianaceae. It contains the following species (but this list is incomplete):

• Exacum affine, Balf.f. ex Regel
• Exacum axillare
• Exacum bicolor
• Exacum caeruleum, Balf.f.
• Exacum pallidum
• Exacum pedunculatum
• Exacum sessile
• Exacum socotranum, Balf.f.
• Exacum tinervium,
• Exacum walkeri