What Is an Automated External Defibrillator? An automated external defibrillator (AED) is a portable device that checks the heart rhythm and can send an electric shock to the heart to try to restore a normal rhythm. AEDs are used to treat sudden cardiac arrest (SCA). SCA is a condition in which the heart suddenly and unexpectedly stops beating.
curl. 1  to form into coils or ringlets, as the hair. 2  to form into a spiral or curved shape; coil. 3  to adorn with, or as with, curls or ringlets. 4  to grow in or form curls or ringlets, as the hair. 5  to become curved or undulated. 6  to coil. 7  to play at the game of curling.
Since curl is the circulation per unit area, we can take the circulation for a small area (letting the area shrink to 0). However, since curl is a vector, we need to give it a direction -- the direction is normal (perpendicular) to the surface with the vector field.