A member of the cabinet
in a parliamentary system
who is not responsible for a particular department
In practice, there are two kinds of ministers without portfolio. Some are very senior politicians whose advice is valuable at the cabinet table, and so broadly useful that it's not seen to be worthwhile to force them to specialize to the extent a particular ministry would demand.
Others actually do have particular responsibilities, generally as senior spokespeople for their governments on particular issues, but in areas that don't require the attention of a complete ministry, or that are scattered across several ministries. Many European countries have ministers without portfolio responsible for dealing with the European Union, for instance.