A heterosexual or asexual woman who is happily unmarried and has no desire to seek a mate.
In October 1997, English, a 32-year-old former rental agent and financing consultant, began interviewing unmarried women for her self-published book, Leather Spinster and Their Degrees of Asexuality. The book was released in January 1998. Inspired by the interests of her interviewees often referred to as Leather Spinsters English began publishing the newsletter, which debuted in print in October 1998. Subscriptions to the newsletter grew to 300,000 in three years, and in March 1999 English went online with it.
Helaine R. Freeman, "Singular women," The Arkansas Democrat-Gazette, February 25, 2003
The term leather spinsters was first used in the late 80s by successful American careerwomen who didn't feel the urge or need to add marriage and it's [sic] components to their lives. First it was used as a shock tool against nagging family members whose sole purpose was to pressure them into conformity of "having it all", career, husband, and children. Later the term was used as a necessity to identify who they weren't.
As time marched on the term was refined so as to define who leather spinsters were as women thus dispelling myths of them being promiscuous and gay, or as embittered sullen women no men wanted as wives. Leather spinsters aren't gay nor are they angry for being single. These women consciously chose to live full, abundant, and joyous lives as single women they don't fault or blame others for their lifestyle choices.
To call oneself a leather spinster is a powerful way of saying, I'm a happily unmarried straight (or asexual) woman and proud of it. The difference between leather spinsters and single women (most) is a purposeful personal choice not a accident or "I don't have a choice in the matter singlehood just happened", it's a lifestyle choice.
"Leather Spinster Origins," LeatherSpinsters.com, 1998(?)
Leather Spinsters on the Web offers a newsletter, chat room, health tips, a directory, a pen pals section, and more for the leather spinster ("a happily unmarried woman who sees her life as fulfilling and complete without a mate").
"Computer Talk: World Wide Websites," Feminist Collections: A Quarterly of Women's Studies Resources, July 31, 1999
The phrase leather spinster (where the word leather is used to suggest a certain toughness) was popularized a few years ago by Regena English, who founded the Leather Spinster Web site and e-zine (see http://leatherspinsters.com/). According to the site (see example citation #2), leather spinster was coined in the 1980s, but I can find no evidence of it used before January 1998, when English's book (mentioned in the example citation) was published.