sidecar

Short definition from Wiktionary

Noun

1. 1
   A one-wheeled attachment to the side of a motorcycle to allow for a separate seat for a passenger or cargo space.
2. 2
   A cocktail made with cognac (or brandy), triple sec liqueur, and lemon juice.
3. 3
   A software component that runs in a separate process from the rest of the software, to provide isolation and encapsulation.