1. a person who takes action or acts; one who does something
Usage: informal; less common than 'actor' in general use
2. in law, a person who brings a legal action or lawsuit
Usage: legal/formal; chiefly British

Examples

• She is an actioner in her own life, always taking steps to improve her situation.
• The actioner filed a complaint against the company for breach of contract.
• He prefers to be an actioner rather than someone who just talks about change.
• The court recognized the actioner's right to pursue damages.
• As an actioner in the community, she organized several volunteer initiatives.