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

See also: optional

Antonym: obligatorily

• penetrator
• Peneus
• penguin
• pengö
• penial
• penicillamine
• penicillate
• penicillin
• penicillin F
• penicillin G
• penicillin O
• penicillin V
• penicillin V potassium
• penicillin-resistant
• penicillin-resistant bacteria
• penicillinase
• penicillinase-resistant antibiotic
• Penicillium
• penile
• penile implant
• peninsula
• peninsular
• penis
• penis envy
• penitence