What is "italo-celtic"

In historical linguistics, Italo-Celtic is a grouping of the Italic and Celtic branches of the Indo-European language family on the basis of features shared by these two branches and no others. These are usually considered to be innovations, which are likely to have developed after the breakup of Proto-Indo-European. It is also possible that some of these are not innovations, but shared conservative features. There is controversy about the actual causes of these similarities. What is commonly accepted is that the shared features may usefully be thought of as "Italo-Celtic forms".