The defined command returns a non-zero value if the symbol is a command, function, or alias, else it returns 0. The symbol must be quoted if it might be an alias. This command is used primarily with the if command when you are programming your own aliases. If the command is being processed in an init file, the quotes must be preceeded by a backslash.
if (defined "isovar") (progn (delete iso1) (plot isovar))
# The following example is read from an init file.