1. showed no electrical activity on a medical monitor, indicating death or cardiac arrest
Usage: medical
2. showed no growth, progress, or activity; remained completely static
Usage: informal

Examples

• The patient flatlined for two minutes before doctors revived him.
• Her heart monitor flatlined during surgery.
• Sales have flatlined for the past three quarters.
• The economy flatlined after the recession began.
• His career flatlined after the scandal broke.
• The stock price flatlined despite positive earnings reports.