The current core definition for Spinster is according to the AHD:

1. A woman who has remained single beyond the conventional age for marrying.
2. A single woman.

Spinster has been equated with "unmarried," in modern English. But historically, the third definition was the only meaning: " 3. A person whose occupation is spinning."

The Middle English version of spinster, spinnestere, very clearly means "female spinner of thread," and is derived from the Middle English infinitive of "spin," spinnen, meaning "to spin,"; it is spin + the feminine suffix -stere. Spinning, the act of twisting and winding or "spinning" flax or wool to make thread is one of the very first technologies of most cultures; spindle whorls, the weights used to steady a drop-spindle so it would spin, have been found on every single continent where humans have lived, and are associated with pretty much every prehistoric culture. The one in the picture above has an ogham c. 8th century inscription which reads "benddact anim," Latin for "a blessing on the soul of L."

In Britain and Europe in general, spinning was mostly done by unmarried girls and young women; so much so that spinster was used in legal documents between c. 1600 to the early 1900s to mean "an unmarried woman." In 1719 in J. Roberts, a woman who identifies herself as "Spinster," explains that "I write myself spinster, because the laws of my country call me so." By 1772, in Oliver Goldsmith's play She Stoops to Conquer Constantia Neville identifies herself as "spinster, of no place at all" (V. i). By the latter half of the eighteenth century, spinster came to mean what the first definition, and current meaning of spinster is: "A woman who has remained single beyond the conventional age for marrying."

I'm not quite sure what happened so that now "spinster" is almost a derogatory term, except that it has something to do with 21st century attitudes about marriage, which increasingly, sound astonishingly like those of the nineteenth century. In an earlier era, as evidenced in Shakespeare's Twelfth Night, spinsters were unmarried women who practiced an important trade:

Duke Orsino

O, fellow, come, the song we had last night.
Mark it, Cesario, it is old and plain;
The spinsters and the knitters in the sun
And the free maids that weave their thread with bones
Do use to chant it: it is silly sooth,
And dallies with the innocence of love,
Like the old age (II. iv. ll. 42–48).

For Shakespeare's Duke Orsino, spinsters are "free maids"; independent women practicing a trade. They earned income from their spinning. My colleague Erika has written here and here about women creating crafts, often astonishing works of art and beauty that require skill, and materials, and hours of labor, and attempting to sell them online. Erika speculates that one reason most crafters, on or offline, earn so little for the quality and skill and art they involve, is because crafts, like spinning, are "traditionally women's work," and is thus devalued. I realize that there are feminists who are attempting to "reclaim" spinster; I wish instead that they would spend money on the work of their peers.