What is a PROX key, Smart Key, or Intelligent Key?

PROX, Smart, Intelligent are all synonyms for remote keyless ignition. Names for this type of system are PROX or PROXIMITY ENTRY, SMART ENTRY or SMART ACCESS, INTELLIGENT ENTRY among others. In moderate to high end vehicle models is where these systems can be found.
How does Remote Keyless Ignition (RKI) work?
The user has a remote that is a key. Usually there is an emergency key blade that slides within the remote for safe keeping but it is not an ignition key. The vehicle has no ignition cylinder for a key to turn – it has push button start. The vehicle can detect the remote while it is in your pocket or purse. When you approach the door it unlocks for you. When you depress the brake pedal and press start the engine starts. On some models, when you walk away the vehicle may lock as well.