It is important to note that while all non-competitive inhibitors bind the enzyme at allosteric sites (i.e. locations other than its active site)—not all inhibitors that bind at allosteric sites are non-competitive inhibitors.  In fact, allosteric inhibitors may act as competitive, non-competitive, or uncompetitive inhibitors.
Many sources continue to conflate these two terms, or state the definition of allosteric inhibition as the definition for non-competitive inhibition.