Cytherean is an adjective

Adjective

In grammar, an adjective is a 'describing' word; the main syntactic role of which is to qualify a noun or noun phrase, giving more information about the object signified....

 meaning pertaining to Cythera (Greek Κύθηρα, also transliterated Kythera or Kithira) , a small island now part of Greece, southeast of the Peloponnesus. It is also an adjective meaning pertaining to the planet Venus

Venus

Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

.

When planetary scientists

Planetary science

Planetary science is the scientific study of planets , moons, and planetary systems, in particular those of the Solar System and the processes that form them. It studies objects ranging in size from micrometeoroids to gas giants, aiming to determine their composition, dynamics, formation,...

 began to have a need to discuss Venus in detail, an adjective was needed. Based on Latin

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 principles, the correct adjectival form of the name would be Venerean. However, this term has an unfortunate similarity to the word venereal as in venereal diseases (related to "Venerean" as martial is to "Martian"), and is not generally used by astronomers. The term Venusian

Venusian

Venusian, Venerean or Venerial may refer to:*Of or relating to the planet Venus*Venusians, hypothetical or fictional beings that inhabit the planet Venus...

 is etymologically messy (similar to saying "Earthian" or "Jupiterian"), and a "cleaner" version was desired.

It was suggested that since Venus had a Greek name, as well as a Roman one, this should be used; however, the adjectival form of Aphrodite

Aphrodite

Aphrodite is the Greek goddess of love, beauty, pleasure, and procreation.Her Roman equivalent is the goddess .Historically, her cult in Greece was imported from, or influenced by, the cult of Astarte in Phoenicia....

 was "aphrodisial", which again was felt to be unfortunately close to "aphrodisiac

Aphrodisiac

An aphrodisiac is a substance that increases sexual desire. The name comes from Aphrodite, the Greek goddess of sexuality and love. Throughout history, many foods, drinks, and behaviors have had a reputation for making sex more attainable and/or pleasurable...

", again evoking matters not directly pertaining to astronomy.

A compromise was reached. In Greek mythology

Greek mythology

Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...

, the goddess Aphrodite

Aphrodite

Aphrodite is the Greek goddess of love, beauty, pleasure, and procreation.Her Roman equivalent is the goddess .Historically, her cult in Greece was imported from, or influenced by, the cult of Astarte in Phoenicia....

 was said to have been born from the sea, from which she emerged on a sea shell at the island of Cythera, and as such was sometimes known as Cytherea. The adjective Cytherean was taken from this name and remained popular in scientific literature for some time.

The term has since fallen out of common use. Venusian is the form most frequently used, with others, including Venerean appearing from time to time; the term Cytherean is now mostly found in older scientific papers, but some scientists still stick to the "tasteful" naming. In addition, the word "Cytherean" as an adjective referring to Venus is often found in science fiction

Science fiction

Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...

of the early and mid 20th century.