An aperception quotient was, in the field of 23rd century psychology, a relative means of measuring one's aperception, a figure that could potentially be used in determining the potential in ones abilities of possessing extrasensory perception.
According to a readout on Dr. Elizabeth Dehner in 2265, she had an aperception quotient of 20/100, with an esper rating of 089. Her ratings were considered "better than average".
Likewise, Gary Mitchell had an aperception quotient of 20/104, with an esper rating of 091. His ratings reached into the next level of "well above average". (TOS: "Where No Man Has Gone Before")