Singers are women with the power to cause changes in the world around with with the power of song and will. They are a relatively new phenomena, one that came about a couple decades after a moderately-sized extraterrestrial object impacted Pern 90 years ago (probably a metallic asteroid given dolphin sonar readings, but there were no witnesses to the impact and the crater is under hundreds of feet of murky water).
Singers seem to have a different biology from ordinary women, despite their outward appearance. They grow very slowly and often reach puberty much later than their peers. They develop their powers very shortly after puberty, accompanied by sudden muteness and sudden, unexplained, and dangerously high fevers that can last up to a week and subside after a mark appears on their body, after which they have full control of their abilities... in theory. In practice, many Singers try to hide what they are for fear of being taken from their homes and can become so stressed out by it that they suddenly lose control of their abilities. This loss of control, called a "Break", tends to be violent and highly destructive, so it's a priority of all Weyrs to find any Singers before they Break. You may notice that this is not helping the base problem.
Singers also seem to Impress at higher rates than other people, and dragonkin seem to generally like them. Additionally, dragons seem to have a stabilizing influence on a Singer's power, as Impressed Singers have more consistent levels of power than their unImpressed peers. No dragon-Impressed Singer has outlived her dragon, but given that some have lost whers to accident and Singers throughout Pern felt the loss... Well, no one wants to find out the consequences.
Physically, Singers tend to be taller than other women. The marks that appear when their power awakens vary in both shape, size, and color among women (the smallest known is a centimeter in diameter, and the largest covers the Singer's back), and no two Singers have identical marks. The size and shape of the mark has no relation to the Singer's strength, despite common belief. The marks are slightly more sensitive than the surrounding skin, but this usually just means that things that would otherwise go unnoticed, like rough cloth, cause mild discomfort. However, marks that appear on the eye or tongue generally make seeing and tasting at ordinary levels extremely uncomfortable if not painful. Additionally, after developing their powers their vocal cords deteriorate and they develop a syrinx
, allowing them to sing harmonies alone with a bit of practice. This also means that they do not require a tongue for speech or singing, although it tends to sound slightly... off.
Singers are all linked on the subconscious level. While this allows for Spells to be transmitted to all Singers capable of using them upon creation, it also creates a sort of mental backdoor that can be exploited by any Singer who figures out how.
It is generally accepted that those who identify as men cannot be Singers.