Raised when a given name is invalid or undefined.

puts foo

raises the exception:

NameError: undefined local variable or method `foo' for main:Object

Since constant names must start with a capital:

Fixnum.const_set :answer, 42

raises the exception:

NameError: wrong constant name answer
Methods
N
T
Class Public methods
NameError.new(msg [, name]) → name_error

Construct a new NameError exception. If given the name parameter may subsequently be examined using the NameError.name method.

Source: show 

static VALUE
name_err_initialize(int argc, VALUE *argv, VALUE self)
{
    VALUE name;

    name = (argc > 1) ? argv[--argc] : Qnil;
    rb_call_super(argc, argv);
    rb_iv_set(self, "name", name);
    return self;
}
Instance Public methods
name_error.name → string or nil

Return the name associated with this NameError exception.

Source: show 

static VALUE
name_err_name(VALUE self)
{
    return rb_attr_get(self, rb_intern("name"));
}
name_error.to_s → string

Produce a nicely-formatted string representing the NameError.

Source: show 

static VALUE
name_err_to_s(VALUE exc)
{
    VALUE mesg = rb_attr_get(exc, rb_intern("mesg"));
    VALUE str = mesg;

    if (NIL_P(mesg)) return rb_class_name(CLASS_OF(exc));
    StringValue(str);
    return str;
}