A nun is a woman bound by religious vows, and canon law draws a precise line within that broad description which ordinary usage tends to blur.
Nun and sister
In the strict canonical sense a nun takes solemn vows and lives in enclosure, given to a hidden life of contemplative prayer, while a sister takes simple vows and lives an active life in the world, teaching, nursing or serving the poor, and though both are commonly called nuns the distinction is real and marks two different forms of the consecrated life for women. The contemplative nun belongs to an enclosed monastery; the active sister belongs to a congregation engaged in works in society.
Entering and its history
Entry is by stages, a period of testing and formation followed by first and then final vows, so the office is assumed gradually and by free profession, as with the monk. The active congregations of sisters are largely a creation of the last few centuries, made possible when the church accepted communities of women taking simple vows without enclosure, a development that opened to women the teaching, nursing and charitable work the enclosed life had precluded. As with the monk, the office is constituted by vows rather than by ordination, and the consecrated life for women, in its contemplative and its active forms, has parallels in the female renunciants and nuns of the Buddhist, Jain and other traditions.