LDS definition: Female Church members ages 18 and older belong to the Relief Society. The Relief Society was founded by the Prophet Joseph Smith on March 17, 1842, in Nauvoo, Illinois. In the days of its founding, it had two main purposes: to provide relief for the poor and needy and to bring people to Christ. The organization continues today, staying true to those original guiding principles as women in the Relief Society meet together on Sunday and in other settings as needed.
Interesting information: The Visiting Teaching Message of the January 2011 Ensign said the following:
Eliza R. Snow recalled the Prophet Joseph Smith teaching that "although the name [Relief Society] may be of modern date, the institution is of ancient origin 1. 1 (1. Eliza R. Snow, "Female Relief Society," Deseret News, Apr. 22, 1868, 81.)
It seems to be saying that Jesus organized an ancient Relief Society. Here's a link to the article Eliza wrote which puts the quote in context:
"FEMALE RELIEF SOCIETY (By Eliza R. Snow.) This is the name of a Society which was organized in Nauvoo, on the 17th of March, 1842, by President Joseph Smith, assisted by Elders Willard Richards and John Taylor. Although the name may be of modern date, the institution is of ancient origin. We were told by our martyred prophet that the same organization existed in the church anciently, allusions to which are made in some of the epistles recorded in the New Testament, making use of the title, 'elect lady.'
"This is an organization that cannot exist without the Priesthood, from the fact that it derives all its authority and influence from that source. When the Priesthood was taken from the earth, this institution as well as every other appendage to the true order of the church of Jesus Christ on the earth, became extinct, and had never been restored until the time referred to above...."
This is something we didn't know before. The concept of a Relief Society type of organization for women in Christ's Church in the First Century A.D. is very remarkable considering that women weren't even allowed to speak in church according to the New Testament:
"Let your Women keep silence in the churches: for it is not permitted unto them to speak; but they are commanded to be under obedience, as also saith the law."
"And if they will learn any thing, let them ask their husbands at home: for it is a shame for Women to speak in the church."
1 Corinthians 14: 34-35