The ankh (pronounced 'ahnk', symbol ☥) was the Egyptian hieroglyphic character that stood for the word ʿnḫ, which means life. Egyptian gods may carry it by the loop, or bear one in each hand crossed over their breast. Latinists interpreted the symbol as a crux ansata, "cross with a handle".
What it was intended to represent remains a mystery to Egyptologists, and no single hypothesis has yet been widely accepted.
Some have speculated that it was a stylized womb. Sir Alan Gardiner speculated that it represented a sandal strap, with the loop going around the ankle. The word for sandal strap was also spelled ʿnḫ, although it may have been pronounced differently. Eliot Carter speculated it could be a primitive representation of human genitalia.
In their 2004 book "The Quick and the Dead", Andrew H. Gordon and Calvin W. Schwabe speculated that the Ankh, Djed and Was symbols have a biological basis derived from ancient cattle culture (and linked to the Egyptian belief that semen was created in the spine), thus:
* the Ankh - symbol of life - thoracic vertebrae of a bull (seen in cross section)
* the Djed - symbol of stability - base or sacrum of a bull's spine
* the Was - symbol of power and dominion - a staff made from a dried bull's penis
The original meaning of this Egyptian symbol is also not known. One suggests that it combines the male and female symbols of Osiris (the cross) and Isis (the oval) and therefore signifies the union of heaven and earth. As a hieroglyph, it likely encompassed a range of meanings depending on its associated hieroglyphs but all of these expressions centered around the concept of life or life force.
Over time, the ankh certainly came to symbolize life and immortality, the universe, power and life giving air and water. "Its keylike shape also encouraged the belief it could unlock the gates of death." The Coptic Christians used it as a symbol of life after death. The ankh has been used in ritual magic.
It also appears to be a 'cross' between a crucifix and the 'christian' (flat) fish symbol which is also represented as determining a point of origin and a vanishing point by drawing two curves around the three main pyramids.
Two ankhs could therefore represent two crossed fishes being a combination of the symbol for Pisces and a crucifix.
In Egyptian art
The ankh appears frequently in Egyptian tomb paintings and other art; it often appears at the fingertips of a god or goddess in images that represent the deities of the afterlife conferring the gift of life on the dead person's mummy. The ankh symbol was often carried by Egyptians as an amulet, either alone, or in connection with two other hieroglyphs that mean "strength" and "health." Mirrors were often made in the shape of an ankh. Sometimes, in art, the Ankh was shown being touched by a god onto a person, which usually symbolized conception.
A similar symbol (♀) was used to represent the Roman goddess Venus. This symbol, known benignly as Venus' hand-mirror, is much more associated with a representation of the female womb. In astrology the same symbol is used to represent the planet Venus, in alchemy to represent the element copper, and in biology to identify the female sex.
Hermeticism is a belief system that is believed to have come out of Egypt and whose beliefs may be able to unify many of the Ankh's meanings. It is unclear whether their beliefs created the ankh or added many meanings, or remain a coincidence. Their concept of God was The All, who purportedly claimed: "Nous, God, being male and female, beginning as life and light, gave birth, by the Word, to another Nous, the Creator of the world;" 
If the concept of the ankh suggesting the joining of the masculine and feminine is correct, with the top opened up to look similar to Ω representing the feminine (genitals) and the bottom shaft being a phallic symbol, then the rest may follow. If God is both male and female, the ankh is a symbol of hermaphroditism and can be representing God. It also can be representing reproduction as both genitalia are pictured, with Nous having given birth. God is also "life and light," making those now synonymous with a symbol of God. God is certainly synonymous with power, and in the Hermetic view, "While All is in THE ALL, it is equally true that THE ALL is in All."  The universe or Cosmos was seen as being the same as The All, making the universe also synonymous with God, and this symbol.
The ankh and the cross
Transitional ankh into Coptic cross found in the Fayuom, 1960s.
Transitional ankh into Coptic cross found in the Fayuom, 1960s.
3D red_cyan glasses recommended for your viewing pleasure
The long standing importance of the Ankh, and its deep symbolism to the dynastic Egyptians, led to it being gradually adopted by the very early Christian church in Egypt (which eventually became the Coptic Church). This is highly significant, as it is almost certainly the genesis of the cross, as the central thematic symbol of the Christian religion. A kind of cross, the ankh, had long been a central religious symbol. It was non-anthropormorphic, not even animal-like. The gods had all been animal faced-human figures. Anknaton's benevolent sun, was the only other symbol that was so esoteric. This cross implied all the "god ideas" that are very infinite in nature. As monotheism is at the core of Christian belief, the ankh seemed a good choice to symbolize the belief in one all powerful God. Over time, the idea that his son had died on a kind of cross, made it seem, all the more appropriate. To other Christians, outside of the ankh's influence, the image the Roman cross of execution was 'shameful" in the manner that a hanging noose would be, or headsman's ax. The association in Egypt of the ankh cross with both God the Father, and Jesus the Son, felt right. Elsewhere, the main Christian symbol at the time had been a stylised alpha, resembling a fish, and therefore known as Ichthys, the Greek word for fish. However, the new "more positive" symbol of a cross eventually spread throughout the Christianized Empire. The distinct circular or "gothic arch-like" upper part of the Ankh was kept well into mediaeval times. The Ankh symbol often was being used as a Christian talisman. The illustration, here, of a Christian 3rd Century bust with a transitional "ankh becoming a cross" , was found in the 1960s in the Fayuom, Egypt, acheological region. It is analogous to the "archaeopteryx fossil"; that's the famous "Dinosaur into Bird", relic, which lends tangible support to the transitional concept. (If you have red-cyan glasses you'll see it in museum grade 3D)
In Unicode, the ankh sign is U+2625 (☥).
In popular culture
The ankh is widely employed in pop culture media as a symbolic device to instantly communicate deep history, arcane life-forces and/or spiritual magic.
* It is sometimes featured in communal alternative events like "Burning Man".
* In the form of a pendant, symbolizing Sanctuary, it was worn by an underground network of Runners attempting to escape their fate in the 1976 science fiction film Logan's Run, based on William F. Nolan's novel of the same title.
* It was worn by David Bowie and Catherine Deneuve as a gold pendant, the bottom part concealing a knife used for blood drinking, in the 1983 vampire film The Hunger, based on Whitley Strieber's novel of the same title.
* It is a notable symbol in Ultima series of computer games, being the symbol most often associated with the Avatar, as well as the Virtue of Spirituality. More recently the Ankh has become one of the frequently appearing items in numerous computer/video games, including Xenogears, Zak McKracken and the Alien Mindbenders, Age of Wonders, Serious Sam The First Encounter, Commander Keen series, Gunbound: World Champion, and Garfield: Caught in the Act.
* It is also widely used in roleplaying fantasy games as a symbol for "holy magic", priest characters, and their equivalents.
* It is suggested that because of his many deaths and reincarnations, the shape of Kenny McCormick in the South Park series is based on the ankh. Kenny always wears an anorak (see Anorankh).
* Vinnie Vincent from the rock group KISS wore an ankh in his make-up design.
* In Neil Gaiman's comic book series Sandman the character of Death always wore an ankh, popular in the Goth subculture that inspired Death's appearance.
* On the anime series Yu-Gi-Oh!, the character Shadi uses the millennium key, which is in the shape of an ankh, to unlock and enter people's minds.
* A broken ankh is the standard symbol of the Sarafan Order in the Legacy of Kain series.
* The Ankh has become a stylish tattoo design. Modern celebrities, such as professional wrestler Christopher Daniels and NBA star Shaquille O'Neil have ankh tattoos.
* It is worn by Karl Sanders of the death metal band Nile, in the music video for the song 'Sacrifice Unto Sebek'.