The term regional flights may be one of the most misunderstood in the aviation industry and there is a simple reason for that. Though the term is often used to describe certain flights, there is no rule in place, or definition, to determine what a regional flight is. Thus, differentiating regional from domestic flights is problematic.
Domestic flights are flights that initiate and terminate within the same country. Regional flights also initiate and terminate within the same country. However, they are thought of as flights over shorter distances. For example, flights between Miami, Florida and Los Angeles, California would be considered domestic flights. Flights between Miami and Orlando, both in Florida, could be considered regional.
Despite the lack of a uniform definition, there are a number of suggestions that would make sense when terming a certain flight a as regional. Regional flights could be any flights that are within a day's drive, or 12 to 15 hours, of the originating airport. This would, by default, limit the area to a certain defined geographic region.
One other possibility is to split the United States up into quarterly regions and say all flights within that area are regional flights. However, arbitrarily drawing lines leads to some problems. For example, a flight originating in St. Louis, Missouri and landing in Louisville, Kentucky may be between two different regions. However, the distance between the two cities is easily reachable in a day's drive. In fact, under this definition, some regional flights may be longer than what would be termed domestic flights. This makes the definition somewhat awkward.
Perhaps regional flights are best defined by the type of plane that is used. This definition would make the most sense for a number of different reasons. Often, smaller jets or prop planes have a limited flight distance, so they must stay within a more limited geographic region than a jumbo jet.
Typically, these smaller jets are called commuter jets or regional jets. So it would logically follow they serve regional flights. Usually, these jets carry less than 50 individuals and have a range on a single tank of fuel of approximately 1,500 miles at most. Thus, if you are taking a flight that has approximately 50 seats or less and you are going less than 1,500 miles, you could reasonably term that as regional.