A real is a number containing a decimal point. Numbers between -1 and 1 must contain a leading zero.
Real numbers are stored in double-precision floating-point format, providing at least 14 significant digits of precision. Note that AutoLISP does not show you all the significant digits.
Reals can be expressed in scientific notation, which has an optional e or E followed by the exponent of the number (for example, 0.0000041 is the same as 4.1e-6). Numbers such as 3.1, 0.23, -56.123, and 21,000,000.0 are all valid AutoLISP real numbers.