Beta plane approximation: variation of Coriolis parameter
The Beta effect describes how the Coriolis force affects fluid motion depending on spatial changes due to the curvature of the Earth. In other words, it describes the variation of the Coriolis parameter with latitude. 

One can model this change exactly or approximate it via Taylor series expansion at a given latitude. The latter option is typically used to make the theoretical analysis of geophysical phenomena possible because the equations are easier to solve. This is the Beta plane approximation, where the Coriolis parameter varies linearly in space.