What the Mercury symbol ☿ means
The Mercury symbol ☿ is a circle with a small cross below it and a crescent, like a pair of horns, on top. It is usually explained as the winged helmet or the caduceus, the herald's staff, of Mercury, the Roman messenger god the Greeks called Hermes. His staff also has its own characters, ⚚ and ☤.
In astronomy ☿ stands for Mercury, the smallest planet and the one closest to the Sun. Modern science writing mostly uses the name instead, but the symbol is still common in astrology, almanacs and older star charts.
Mercury in astrology
Astrologers use ☿ for the planet that rules two signs, ♊︎ Gemini and ♍︎ Virgo. About three times a year, Mercury seems to move backward across the sky for a few weeks, an optical effect of the small, fast planet overtaking Earth on its inner orbit. Charts mark this as retrograde with ℞, so ☿℞ is common shorthand for "Mercury retrograde".
The planet also gave its name to a day of the week. Wednesday was Mercury's day in Latin, which still shows in French mercredi, Italian mercoledì and Spanish miércoles.
Mercury in alchemy
For alchemists, ☿ meant the metal mercury, also called quicksilver because it is a silvery liquid at room temperature. It was one of seven metals paired with the seven classical planets: ☉ gold, ☽ silver, ☿ quicksilver, ♀︎ copper, ♂︎ iron, ♃ tin and ♄ lead.
The Renaissance physician Paracelsus built his teaching on three principles: mercury ☿, sulfur 🜍 and salt 🜔. Chemistry later gave the element the symbol Hg, from the Latin hydrargyrum, "water silver", which is what belongs in a modern formula.
☿ as a non-binary and intersex symbol
Because ☿ combines the circle and cross of ♀︎ with a crescent on top, many people use it as a symbol for intersex, androgynous and non-binary identities. It sits alongside ♀︎ (female), ♂︎ (male), ⚥ (male and female) and ⚧︎ (transgender); the non-binary symbol page covers alternatives such as ⚲.
Unlike the zodiac signs, ☿ is not an emoji, so it never turns into a colored icon on phones. To type it, search "Mercury" in the Mac Character Viewer (Control + Command + Space), type 263F and press Alt + X in Word, or use \mercury from the wasysym package in LaTeX. More options are on the gender symbols page.