Ronald Hutton in his book Triumph of the Moon says that the origins of the word "pagan" has long been accepted as "paganus" which means "rustic" and symbolised the dominance of Christianity as an urban and sophisticated religion over a backwards old countryside faith.
However he also goes on to say that this meaning had never been proved and French academic Pierre Chuvin challenged the meaning, proposing instead that it referred to those who preferred the faith of the "pagus", the local unit of government and by association the older rooted religions (1)
The difficulty in attempting to pin a modern definition down is that many who would consider themselves pagan will argue about what that means. As Cassiel said, "pagan" is most widely accepted as being an umbrella term to cover a number of reconstructed indigenous religions and earth-based paths, such as Druidry, Heathenry, Asatru, Hellenism, Wicca , Shamanism and so on. On their website, the Pagan Federation define a pagan as "A follower of a polytheistic or pantheistic nature-worshipping religion." (2) The problem arises however when those within the various groups don't consider themselves pagan, despite being labelled as such. Many Heathens and Hellenists in particular prefer the term "polytheist" over "pagan".
The same problem arises within Wicca, as some Wiccans would consider themselves pagan and others wouldn't. A Traditional Wiccan viewpoint of Wicca is that it is an experiential "Initiatory Mystery Tradition" (3) in a similar vein to the Eleusinian or Bacchic Mysteries and comprising of magick (usually witchcraft), mysticism and spirituality. Traditional Wicca has a very precise set of rites, hierarchy, and is initiatory. An Eclectic Wiccan view is that Wicca is a nature-based pagan religion, tends to have a looser "do what works for you" feel about it and doesn't have to be initiatory, though some modern covens have established their own lineages.
Basically, it depends on what type of Wiccan you ask.
1: Triumph of the Moon, Ronald Hutton
2: The Pagan Federation
3: Towards the Wiccan Circle, Sorita d'Este