What is a catalytic domain?
The catalytic domain is roughly referring to the same part of the protein as an active site and may be used interchangeably in some circumstances:

See here, where "catalytic domain" redirects to "active site."

There is, however, a difference:

From a free online medical dictionary: 


  active site: 
  the place on the surface of an enzyme where its catalytic action occurs.


From Wikipedia:


  catalytic domain: structurally conserved (tertiary) structure containing the catalytic function of the protein.


To further clarify: the catalytic domain is the 3D structural independent folding unit of the protein that contains an active site, while the active site (aka "catalytic site) is the area located within the catalytic domain where substrate molecules bind and undergo a chemical reaction.