Definition of vertical fin in English:

vertical fin

noun

Zoology
  • Any of the unpaired fins in the midline of a fish's body, i.e. a dorsal, anal, or caudal fin.

    • ‘But the first thing he saw as he surfaced was not a dolphin but the vertical fin of a great white.’
    • ‘At hatching the larvae are quite advanced with pigmented eyes, well-developed jaws, digestive tracts, and vertical fin folds.’