Butanone or butan-2-one?
The preferred IUPAC name is butan-2-one. This is explicitly mentioned in the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book).



Generally, the complete information about a structure is explicitly given by the name and does not rely on any implied information. Nevertheless, the practice of omitting locants when there is no ambiguity is widespread in general nomenclature. For preferred IUPAC names, however, locants are omitted only in a few exceptional cases described in Subsection P-14.3.4 of the Blue Book:


P-14.3.4.1 terminal locants
P-14.3.4.2 the locant ‘1’
P-14.3.4.3 where there is only one kind of substitutable hydrogen
P-14.3.4.4 when no isomer can be generated by moving suffixes or prefixes from their position to another or by interchanging them between two different positions
P-14.3.4.5 when all substitutable positions are completely substituted
P-14.3.4.6 when all substitutable hydrogen atoms have the same locant.


None of these cases applies to butan-2-one. Therefore, the locant cannot be omitted in the preferred IUPAC name.