Menopause is usually a natural change; it typically occurs in women in midlife, during their late 40s or early 50s, signaling the end of the fertile phase of a woman's life.
Menopause is commonly defined by the state of the uterus and the absence of menstrual or "periods", but it can instead be more accurately defined as the permanent cessation of the primary functions of the ovaries. What ceases is the ripening and release of ova and the release of hormones that cause both the build-up of the uterine, and the subsequent shedding of the uterine lining (the menses or period).
The transition from a potentially reproductive to a non-reproductive state is normally not sudden or abrupt, occurs over a number of years, and is a consequence of biological ageing. For some women, during the transition years the accompanying signs and effects (including lack of energy, hot flushes, and mood changes) can be powerful enough to significantly disrupt their daily activities and sense of well-being. In those cases various different treatments can be tried.
Medically speaking, the date of menopause (in a woman with an intact uterus) is the day after the final episode of menstrual flow finishes. "Perimenopause" is a term for the menopause transition years, the time both before and after the last period ever, while hormone levels are still fluctuating erratically. "Premenopause" is a term for the years leading up to menopause. "Postmenopause" is the part of a woman's life that occurs after the date of menopause; once a woman with an intact uterus (who is not pregnant or lactating) has gone a year with no flow at all she is considered to be one year into post menopause.