1. Showing sustained enthusiastic action with unflagging vitality
   "unflagging pursuit of excellence"
   - indefatigable, tireless, unwearying 
2. Unceasing
   "unflagging courtesy"
   - unfailing

See also: constant, energetic

• niello
• Niels Abel
• Niels Bohr
• Niels Henrik Abel
• Niels Henrik David Bohr
• Nielsen
• Niemann-Pick disease
• nierembergia
• Nierembergia frutescens
• Nierembergia repens
• Nierembergia rivularis
• Nietzsche
• nifedipine
• niff
• niffy
• nifty
• nigella
• Nigella damascena
• Nigella hispanica
• Nigella sativa
• Niger
• Niger franc
• Niger River
• Niger-Congo
• Niger-Kordofanian