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

See also: constant, energetic

• Symplocus tinctoria
• sympodial
• symposiarch
• symposiast
• symposium
• symptom
• symptomatic
• symptomatically
• symptomatology
• symptomless
• synaeresis
• synaesthesia
• synaesthetic
• synagogue
• Synagrops
• Synagrops bellus
• Synanceja
• Synanceja verrucosa
• synapse
• synapsid
• synapsid reptile
• Synapsida
• synapsis
• synaptic
• Synaptomys