Agenothree Pernese word for Nitric Acid, or HNO³. This chemical was loaded into portable tanks and used by the riders in the Queen's Wing or thread fighting ground crews to destroy any thread that reached the ground before it could burrow into the soil.
Belior The name of the closest of Pern's two moons (the other is Timor).
Between An area of nothingness and sensory deprivation between "here" and "there." Dragons and their riders spend moments between as they travel between two points in space and time. It is extremely cold between.
Crawlers A sort of small, six-legged, gecko looking creature that crawls along walls and even ceilings with their sticky little feet. They have a round stinging mouth and no eyes. Humans and dragons loathe them, fire lizards love to eat them.
Day Sisters/Dawn Sisters The Pernese name for the Yokohama, Buenos Aires, and Bahrain. The term was mostly used after it was forgotten that they were spaceships, and they were assumed to be just a trio of unusually bright stars.
Deadglow a numbskull, stupid. Derived from "glow."
Dragon's Tongue A native succulent Pernese plant which produces a gel similar to Terran aloe.
Dragonet Original name for fire-lizards Also refers to young dragons
Drudge A male or female worker engaged in the lowest levels of manual labor or upkeep of a Weyr, Crafthall or Hold.
Eridani The Eridani were an alien people expert in genetic manipulation. They trained Kitti Ping Yung and invented Mentasynth
"Fardles" or "Fardling" Pernese oath
Fellis A native Pernese plant which produces a juice that is used as a soporific drug.
Firefly A small winged beetle whose abdomen glows with a luminescent light. Looks a bit like the Terran firefly, but tends to glow in different colors.
Firestone A phosphine bearing rock found on Pern. Dragons and fire lizards can chew it to produce phosphorous gases which are ignited to burn Thread out of the sky.
First Crossing The "First Crossing" is a term that refers to the 15-year, one-way journey that the original Pern colonists made from Earth to Pern. Most of the colonists were in suspended animation for the entire 15-year trip. The crew of the colony ships were awake for 5-year shifts, and asleep the other 10.
Flamestone A highly volatile variant of firestone, capable of exploding into flame upon contact with moisture. Mistakenly used as firestone throughout the Second Interval.
Fly-by A small flying insect much like a gnat.
Grub A type of worm or millipede that has spread throughout the southern continent. They are known both for their eating of Thread and their beneficial effects on the plants around them.
Hold A hold is where all the "normal" folk of Pern live. Originally carved out of Thread-defensive rocky cliffs, they grew to include seats of government, commerce, and agriculture. They were led by Lord Holders, to whom citizens looked for guidance and leadership.
Mentasynth Mentasynth is a form of advanced bio-engineering developed by the Eridanis. The Mentasynth process results in enhanced mental abilities of certain animals resulting in increased intelligence and empathy. The abilities that result from Mentasynth are genetically transmitted.
Needlethorn A native succulent Pernese plant whose hollow thorns can be used as hypodermic needles.
Numbweed A native Pernese plant that contains a strong analgesic which is made into a cream that numbs small wounds completely and dulls the pain of larger wounds.
Red Star Not a star, but a planet, the Red Star was a planet that was attracted into the Rukbat System only a few million years before the Pern expedition. Its orbit was almost parabolic in shape. Because of its orbit, it was conjectured by the original Pern colonists that the Red Star might eventually escape Rukbat's orbit or fall into the sun. It was noticed but ignored by the EEC team who explored Pern and declared it suitable for colonization. The colonists, upon arrival in the Rukbat system, speculated that they might see some spectacular meteor showers in about eight years time when the cometary trail crossed Pern's orbit.
Rukbat The name of the sun around which Pern orbits. Rukbat is found in the constellation Sagittarius the Archer.
"Scorch it" Pernese Oath
Second Crossing The Second Crossing refers to the relocation of Pern's colonists from the Southern Continent to the Northern Continent during their 10th turn on the planet. The term "Second Crossing" was used since it was the second time that the colonists relocated their base of civilization, the "First Crossing" was the 15 year voyage from earth. The relocation to the Northern Continent was necessitated by volcanic activity that threatened the colony. The most suitable site for their relocated colonial headquarters was located on the Northern Continent which was less susceptible to volcanic activity and contained large number of caves that would offer natural shelter from threadfall.
Spinner Native species of Pernese spiders. There are also imported species descended from ones brought by the colonists whose silk can be gathered and spun like silkworm silk, though not quite as fine. Called gossamer spinners.
Springs Insects that hang in spiral loops until they find someone or something to cling to. They have an irritating, prickly bite.
"Taking a short dragon-ride" A euphemism for the ending of an unwanted pregnancy. This arose because the extreme cold of between can cause spontaneous abortion, but did require sensible a convalescence. This process of abortion was accepted on Pern, at least among dragonriders. As a result of this risk female green riders would fight Fall with the queen's wing (where they would not need to jump between as frequently) when they were pregnant and wanted to keep the baby. According to Telgar Weyrwoman Zulaya there had only been a few of these between-abortions in her time as Weyrwoman (end of First Interval/beginning of Second Pass).
Timor The name of the moon which is furthest from Pern (the other moon is Belior).
Trundlebug A useful insect that eats parasites, turns the soil and acts as a pollinator. They have the most elaborate color camouflage of all the insects found on Pern and come in many colors.
Tunnel Snake Tunnel snakes are reptilian vermin that infest Pern. Unlike Terran serpents, tunnel snakes possess six limbs which some use to burrow and dig. Others are adapted to survive an amphibious life, with either webbed feet or paddles. Only a few species of tunnel snakes are edible, as the colonists learned early on. Some species also have a venomous bite, that could prove fatal if left untreated.
Turn Pernese word for 'year'
VTOL A sort of furry insect with double pairs of wings. The discoverers of these insects named them VTOLS (for "Vertical Take-Off and Landing").
Weyr Either a place that is home to many dragons (Weyr) or the resting place of an individual dragon (weyr).
Watchdragon the dragon whose rider has pull watch duty on the Weyr roster. A watch is generally four hours long. Essentially Weyrs are military camps. Sentries are part of the ethos. During a Pass, they watch for any chance of erratic Fall of Thread, or for anyone entering or leaving the Weyr 
Weyrwoman The rider of a gold dragon. When capitalized, the rider of the senior queen who is responsible for managing the Weyr.
Wherhide Pernese term for leather.
Wherries Wherries are large avian creatures with 6 limbs, a strong rear pair for walking and grasping; the forelimbs are used for rending and tearing, while the middle set of limbs are large leathery wings. Unlike Terran birds, the wherry is covered in a soft fur like down. Wherries are used as food stock, and their fatty flesh will often be an indication of where it fed, picking up a fishy taste when it feeds by the sea, and a nutty taste when it has fed on grains from the plains. They were named for their resemblance to wherries, a form of flat-bottomed barge used on Earth, by the EEC survey team. It is unclear in the books if there are different species of wherries.
Withies are an aquatic plant found in the Northern Continent's many rivers. In the Ninth Pass it was common for an entire Hold's population to go gathering them in spring. They were used in basket-weaving.