What is Air Taxi?

Legal Definition
An air taxi is a small commercial aircraft which makes short flights on demand.

In 2001 air taxi operations were promoted in the United States by a NASA and aerospace industry study on the potential Small Aircraft Transportation System (SATS) and the rise of light-jet aircraft manufacturing.
-- Wikipedia
Legal Definition
A small aircraft with 20 or less passengers that flies for 250 miles to get the occupants to their commercial flights.