silver bullet

Short definition from Wiktionary

Noun

1. 1
   A bullet made of silver, usually with reference to the folkloric belief that such bullets are the only weapons which can kill a werewolf, vampire, or other monster.
2. 2
   Any straightforward solution perceived to have great effectiveness or bring miraculous results.
3. 3
   A classic dry martini cocktail.
4. 4
   A thermometer, especially one used for rectal temperature readings.