Buoyancy is the force caused in the gravitational field towards the direction opposite to gravity.
Followings are typical examples:

1. - Force caused by density difference due to change in fluid temperature (below top)
2. - Force applied from fluid being pushed by an object floating on a fluid (below bottom)

 

 

Basic Course of Thermo-Fluid Analysis: Chapter 4 Basics of Heat
4.1 Temperature and heat