ASPS:
Absolute String Plane Stiffness, or String Bed Stiffness. This is the
stiffness of the entire string bed and is the only quantifiable
"tension" number.
Reference Tension:
This is, normally, the tension number you communicate to your racquet
technician and what the stringing machine setting will be.
COP:
Center of Percussion, which is calculated for each racquet and indicates
the location on the string bed where there is no vibration as the
result of ball impact.
Flex:
This is the bending stiffness of the racquet as if hitting a ball.
Swing Weight:
This is the dynamic weight of the racquet and the measure of
maneuverability and power. All things being equal, the higher this
number the more power potential in the racquet.
Recoil Weight:
This is the swing weight around the center of mass of the racquet and
indicates racquet stability. All things being equal the higher the
number the better.
Additional terms will be added as they become necessary to better understand racquets and strings.