These are all generic terms used to describe one hour of engineering continuing education credit for various states.
PDH means Professional Development Hours and is used by most states.
CPD, used in Florida, Ohio, Louisiana, and other states, means Continuing Professional Development credits.
The term CPC or Continuing Professional Competency credits is used in New Jersey, Maryland, and other states.
Other states may have different terminology. Although the terminology changes from state to state, the meaning is the same. Each PDH, CPD, or CPC credit is equivalent to one hour of training time.