Simplified Common Lisp reference
isqrt
Symbol class: Mathematics, Arithmetics, Logic and Comparisons
Syntax:
Symbol type: function
isqrtnumber => integer value
Argument description:
number positive integer

ISQRT function computes integer part of square root of number. See also SQRT. 

(isqrt 10) => 3
(isqrt 4) => 2
Function indexFull documentation for isqrt (HyperSpec)