A group 18 element is any chemical element from the last column of the standard periodic table.
For the first six periods, the group 18 elements are exactly the noble gases. However, the seventh member of group 18 (the synthetic element ununoctium) is probably not a noble gas.
Group 18 was previously called 'group 8A' or 'group 0'. According to the classical shell model for electrons, the group 18 elements have a fully filled outer shell, rendering them inert to most chemical reactions. This holds true for the first six elements of this group (though they tend to become slightly less inert with increasing periods). For the seventh period group 18 element (ununoctium), this "nobility" is predicted to break down due to relativistic effects.
