First meaning: Pagans consist of Wiccans and other Neopagans:
We recommend that this should be the primary definition of "Pagan," for the simple reason that many Wiccans and other Neopagans embrace the term for themselves. "Paganism" in this sense refers to a range of spiritual paths. These are generally Neopagan religions based on the deities, symbols, practices, seasonal days of celebration and other surviving components of ancient religions, which had been long suppressed. For example:
The Druidic religion is based on the faith and practices of the ancient Celtic professional class;
Followers of Asatru adhere to the ancient, pre-Christian Norse religion;
Wiccans trace their roots back to the pre-Celtic era in Europe
Other Neo-pagans follow Hellenismos (ancient Greek religion), Religio Romana (ancient Roman religion), Kemetism (ancient Egyptian religion) and other traditions

Some Neopagan religions, like Wicca and Druidism, are Earth centered: emphasizing living in harmony with the Earth and observing its cycles. Others, like Hellenismos and Religio Romana, are deity centered.
Some typical quotations which demonstrate this meaning of "Pagan" are:

"Witchcraft, or Wicca, is considered part of the occult, but has little relationship to Satanism. Wicca is pagan (pre-Christian, as opposed to anti-Christian) and is currently gaining popularity." 1

Witches do not worship the devil...Witches are more interested in magical arts and the divinity of nature...Wiccans are considered pagans because they worship several nature gods instead of a single god." 2

"The World Christian Encyclopedia estimates 6 million Americans profess to be witches and engage in practices like these. They are a sub-group of over 10 million persons the encyclopedia says call themselves pagans, who practice "primitive" religions such as Druidism, Odin worship and Native American shamanism." 3

In this sense, "Pagan" refers to a group of religious traditions, and should be capitalized, as Christianity, Islam and Judaism are.

Second meaning: Pagans are people to hate:

Religious and social conservatives sometimes use "Pagan" as a general purpose "snarl" word to refer to cultures or religions that are very different from the speaker's. There is no general consensus as to meaning. It can be seen directed at any religious or cultural group that the speaker hates. Some examples:

Dr. John Patrick, professor at the University of Ottawa, Canada was discussing at a conference the number of abortions performed worldwide. He said: "Gods and goddesses are beginning to re-inhabit the Western world. Infant sacrifice -– there are 52 million a year. It is paganism." 4
Dr. Richard Swenson, director of the Future Health Study Center. said at the same conference: "We went into post-Christian and neopaganism very quickly...We want the culture to change, we want some spiritual sanity, but we need to understand that this is a pluralistic and even neopaganist society." 4
Jerry Falwell appeared as a guest on Pat Robertson's "700 Club" program on 2001-SEP-13. He said that God became sufficiently angry at America that he engineered the terrorist attack on New York City and Washington-- presumably to send Americans a message. He said: "I really believe that the Pagans, and the abortionists, and the feminists, and the gays and the lesbians ...all of them who have tried to secularize America - I point the finger in their face and say 'you helped this happen.' " Pat Robertson responded: "Well, I totally concur..." 15 [We asked Falwell's office via repeated Emails to tell us exactly to whom he was referring with the word "Pagans." They declined to respond. Since he died on 2007-MAY-15, we suspect that the question will never be answered.] More details.

Third meaning: Pagans are ancient polytheists:
The term "Pagan" is sometimes used to refer to ancient polytheistic religions. The Cambridge Advanced Learner's Dictionary defines "pagan" as: "belonging to a religion which worships many gods, especially one which existed before the main world religions." 18
The Hebrew Scriptures (Old Testament) contains many references to the societies surrounding the Israelites -- Babylonians, Canaanites, Philistines, etc. These are commonly referred to as Pagans:

. There are allegations that these societies engaged in human sacrifices:
1-II Kings 3:26-27: "...the king of Moab...took his eldest son that should have reigned in his stead, and offered him for a burnt offering upon the wall."

2-Psalms 106:37-38: "Yea, they sacrificed their sons and their daughters unto devils, And shed innocent blood, even the blood of their sons and of their daughters, whom they sacrificed unto the idols of Canaan: and the land was polluted with blood."

. Their altars were often referred to as "high places:"
II Kings 16:4: "And he sacrificed and burnt incense in the high places, and on the hills, and under every green tree."

. Surrounding tribes were viewed as committing idolatry by worshiping golden images of animals:
II Kings 17:16: "And they left all the commandments of the LORD their God, and made them molten images, even two calves, and made a grove, and worshipped all the host of heaven, and served Baal."

Some current examples of this usage are:
Referring to sun wheels and obelisks: "...These symbols of pagan sun worship were associated with Baal worship, or Baalim, which is strongly condemned in scripture. So why are they so prevalent in the Roman Catholic Church, if they are associated with paganism and apostasy?" An anti-Catholic essay on a conservative Protestant Christian web site. 5
Ancient faiths of ancient Celtic, Egypt, Greece, Norse, Rome, and other cultures are frequently referred to as Pagan religions. Even though many of these religions had strict social and sexual behavioral codes, their followers are often portrayed as hedonist and immoral:
1 Peter 4:3: "For the time past of our life may suffice us to have wrought the will of the Gentiles, when we walked in lasciviousness, lusts, excess of wine, revellings, banquetings, and abominable idolatries."

Fourth meaning: Pagans follow Aboriginal religions:
Paganism is occasionally used to refer to animistic, spirits-and-essences filled belief systems. These are based upon direct perception of the forces of nature and usually involves the use of idols, talismans and taboos in order to convey respect for these forces and beings. Many native, aboriginal religions fit this definition.
Fifth meaning: Pagans are followers of non-Abrahamic religions:
A rare use of "Pagan" is to describe a person who does not follow an main Abrahamic religion. That is, their faith does not recognize Abraham as a patriarch. The individual is neither Christian, Muslim, Baha'i nor Jew. This includes Agnostics, Atheists, Buddhists, Hindus, Humanists, Taoists, etc. About 45% of the people of the world are Pagans, by this definition.

Sixth meaning: Pagans don't belong to any of the main religions of the world:
The Cambridge Advanced Learner's Dictionary gives an alternative definition of the word "pagan" as: "relating to religious beliefs that do not belong to any of the main religions of the world" 18 This definition is rather vague, because it does not describe how a "main religion of the world" is defined. If it is any religion with more than, say, 1% of the world's population (i.e. 60 million members, then: Aboriginal religions, Atheism, Buddhism, Christianity, Hinduism, Islam, Taoism would be non-pagan, whereas Judaism, Sikhism, Confucianism, the Baha'i World Faith, Wicca, Zoroastrianism etc. would be pagan. I doubt that many members of the latter religions would be happy with their classification.

Seventh meaning: Pagans are Atheists, Agnostics, Humanists, etc:
The term "Pagan" was widely used by Atheists, Agnostics, Humanists, etc. to refer to themselves. The word was also used by others to describe these groups. The usage dropped after the rise of Neopaganism in the middle of the 20th century, and is rarely seen today.