Ok this is the Wikipedia definition.
"All-wheel drive (AWD) is often used to describe a "full time" 4WD that may be used on dry pavement without destroying the drivetrain (It should be noted that "Full-Time" 4WD can be disengaged and the center differential can be locked, essentially turning it into regular 4WD. On the other hand, AWD cannot be disengaged and the center differential cannot be locked.)"
center differential = transfer case