1. a person whose heart has stopped beating; a patient showing no heartbeat on a cardiac monitor
Usage: medical; informal
2. something or someone that shows no growth, improvement, or change; a stagnant performer or product
Usage: informal; business/sports

Examples

• The paramedics rushed to treat the flatliner in the emergency room.
• The patient was a flatliner for three minutes before the defibrillator revived him.
• That movie was a flatliner at the box office despite the big marketing budget.
• The company's sales have been a flatliner for the past two quarters.
• His performance this season has been a flatliner compared to last year's success.