Dead reckoning is a “backup” feature that allows a vehicle navigation system to continue estimating your location in situations where it’s unable to receive signals from GPS satellites.
The location calculated by dead reckoning usually become less accurate the longer you go without GPS reception.
Dead reckoning functionality is included with some GPS models from Garmin and TomTom. It's not usually available in entry level models.
TomTom uses the phrase "assisted satellite navigation" as the label for its dead reckoning functionality.