weather vane

Short definition from Wiktionary

Noun

1. 1
   A mechanical device rotating around one axis and attached to an elevated object such as a roof for showing the direction of the wind.
2. 2
   An indicator; something that reflects what the current situation is.
3. 3
   A person or organization that changes their attitude and position based on the prevailing conditions rather than displaying any conviction.
4. 4
   Alternative form of weather vane.