The Berry mechanism refers to the facile interconversion of axial and equatorial ligand in  types of compounds, e.g. D3h-symmetric  (shown). Finally, in a formal sense, the term pseudorotation is intended to refer exclusively to dynamics in symmetrical molecules, though mechanisms of the same type are invoked for lower symmetry molecules as well.
The Berry mechanism describes the effortless transition between axial and equatorial ligands in certain compound types, such as D3h-symmetric ones (as depicted). In a formal sense, pseudorotation is intended to specifically refer to dynamics within symmetrical molecules. However, similar mechanisms are also utilized for lower symmetry compounds.