The variable `pi` is defined to be pi, see
print pi
Additionally, gnuplot may define some variables under various operations.
Working with interactive terminals with `mouse` functionality defines variables with names that begin "MOUSE_", see variables for details.
Further, there are several "read-only" variables that begin "GPVAL_", like GPVAL_TERM, GPVAL_X_MIN, GPVAL_X_MAX, GPVAL_Y_MIN,... Type `show variables all` to display their list and values. Values related to axes parameters (ranges, log base) are values used during the last plot, not those currently `set`.
The fit mechanism uses several variables with names that begin "FIT_". It is safest to avoid using such names. "FIT_LIMIT", however, is one that you may wish to redefine. Under `set fit errorvariables`, the error for each fitted parameter will be stored in a variable named like the parameter, but with "_err" appended. See the documentation on fit for details.