1. In an optional manner
   "this rule is applied optionally"

See also: optional

Antonym: obligatorily

• prolixness
• prolocutor
• Prolog
• prologise
• prologize
• prologue
• prologuise
• prologuize
• prolong
• prolongation
• prolonge
• prolonge knot
• prolonged
• prolonged interrogation
• prolusion
• prolusory
• prom
• promenade
• promenade concert
• promenade deck
• promethazine
• Promethean
• Prometheus
• promethium
• prominence