1. (of weapons, substances, or conditions) causing death
   "lethal weapon"; "a lethal injection";
   - deadly
    
2. Resulting in widespread death or destruction
   "A lethal disaster claimed hundreds of lives"
    
3. [informal] Extremely effective or formidable
   "His lethal combination of speed and skill made him unstoppable"

See also: fatal, killing

Encyclopedia: Lethal