Is there any particular criterion that makes a CPU be called "16 bits" CPU for example
Short answer:  There is no universally accepted definition.  

Less short answer:  If the CPU supports all of the basic primitives on a 16-bit datatype, then it would probably be considered sixteen bits by the majority of users.

This has been a "holy war" since 1976 or so, and there is no "right" or "wrong" answer.  Was the 8088 sixteen bits?  Probably.  So, was the Z80?  It had some sixteen bit math, and an 8-bit databus.  (Probably not -- the Z80 had no native 16-bit logical instructions, only add and subtract).

The question surged again when the 68000 with its 32-bit registers, and rich set of 32-bit operations appeared, but an internal 16-bit ALU and an external 16-bit databus (and then just to throw MORE confusion, the 68008 variant, with an 8-bit databus).