Activator may mean:

• Activator (proteomics), a type of effector that increases the rate of enzyme mediated reactions
• Activator (genetics), a DNA-binding protein that regulates one or more genes by increasing the rate of transcription
• Sega Activator, a motion-sensing controller for the Sega Genesis
• Activator (day after partying), The phenomena experienced when one beer or drink or drug taken the morning or afternoon after having been intoxicated the night before returns the sensation of being intoxicated. eg "This activator brought me right back to how I felt at the club last night."