The following are the relevant commands:

DEFINE SYMBOL

usage:

        DEFINE SYMBOL symbol_name = string

SHOW SYMBOL

usage:

        SHOW SYMBOL/ALL
        SHOW SYMBOL symbol_name
        SHOW SYMBOL partial_name

CANCEL SYMBOL

usage:

        CANCEL SYMBOL/ALL
        CANCEL SYMBOL symbol_name

Legal symbol names must begin with a letter and contain only letters, digits, underscores, and dollar signs.

To invoke symbol substitution—the replacement of the symbol name with its (text) value—within a Ferret command include the name of the symbol preceded by a dollar sign in parentheses.

For example,

yes? DEFINE SYMBOL hi = hello everyone
yes? MESSAGE ($hi)              ! issues "hello everyone" msg

It is also possible to nest symbol definitions, as the following commands illustrate:

yes? DEFINE SYMBOL label_2 = My test label
yes? DEFINE SYMBOL number = 2
yes? SAY ($label_($number))
    My test label