From a human rights standpoint, we tend to avoid defining people, but I think what defines the Romani peoples is that there's a common history, a common language and a common cultural heritage. In terms of the history of Roma, it's actually a history of discrimination and persecution. That's part of the reason that Roma are actually a global diaspora. Roma came through Europe in the 12th century originally from northern India, and are now spread across the world. There are Roma populations in the Middle East, in Australia, and in Latin America as well.
In terms of definition, there are common traits. The common traits are in fact the common history and cultural heritage of Roma, but it's very difficult to restrict the group of people to a definition. There are many subgroups throughout the Roma.