What’s the definition of a regional carrier again?

One defined by some 1980’s monetary yardstick?

Is the Americas now considered a region?

There are regional sized aircraft with longer range than an A300. Which one is the regional aircraft?

Most regionals carry passengers who bought a ticket through another carrier. Many passengers on mainline carriers buy tickets through other carriers on codeshares. Similar concept why the different name?

Could it be that a regional airline has simply become an expression for a bottom tier scheduled 121 operator for whom passengers, employees, and management alike accept lower standards? Why? Because “regional”

Why wouldn’t we reject this term altogether?