What the heck is Premenopause, Perimenopause and Postmenopause?

As defined by the World Health Organization, menopause is the permanent cessation of menstruation due to the loss of ovarian follicular activity . This definition uses both a symptom that can be identified by a woman (the end of menstruation) and a sign that can be measured (the loss of follicular activity results in changes in levels of hormones). Investigators have generally agreed to define menopause as the last menstrual period followed by at least twelve months of amenorrhea (no menstrual bleeding). The advantage of this definition is that it identifies a single, measurable variable within the climacteric transition. This definition makes it possible to compute median or mean ages at menopause for inter- and intrapopulation comparisons. The definition also allows one to delineate a clinically normal range in age at menopause (for example, ages forty to sixty). Finally, this definition enables clinicians to identify
women who are postmenopausal for medical “management.” But while thelast menstrual period is a clinically useful marker of an event, the average woman’s sense of the process of the menopausal transition is better described by the term “perimenopause,” a gray, difficult-to-define time period during which a woman wonders if each period of bleeding is the last.

K. I know thats too complicated so take this:

Stages and Definitions of the Menopause Transition

Premenopause: Regular cycling. Having experienced a menstrual period
during the two months prior to study.

Perimenopause: Irregular cycling. Having experienced a menstrual period
from three to eleven months prior to study.

Postmenopause: Having experienced the last menstrual period at least
twelve months prior to study

That said, women still perceive menopause to be a marker for the end of childbearing because most women have no other “window” into the state of their ability to conceive. The end of menstruation can, therefore, be an emotionladen event. Some women react to the cessation of menstruation with relief (no more birth control); others describe deep sadness because they can no longer bear children.

Menopause: Is it a one time event or an ongoing process?

Menopause is often treated as a onetime, life history event for cross-species and cross-cultural comparisons, as well as a medical event (a marker of estrogen decline). Women themselves, however, usually experience menopause as a more gradual, transitional process.

Menopause as a Process
The last menstrual period is an event that occurs within the transition from a reproductive to a postreproductive stage of life. In most countries this transition period is called the climacteric, but in the United States researchers speak more commonly of the “perimenopause”.
In general, the climacteric or perimenopausal transition begins when menstrual periods become irregular and ends one year after the last menstrual period. The word “climacteric” comes from the Latin word climactericus, meaning “of a dangerous period in life,” or from the Greek word klimakterikos, from klimakter, meaning “a dangerous point, the rung of a ladder.” “A dangerous period in life” may seem less favorable than “perimenopause”; however, because the word “climacteric” is used more often internationally, I employ both terms.

Menopause is related to the climacteric much as menarche is related to puberty. In other words,
menopause is the most prominent signal that the female body is exiting the reproductive period, just as menarche is the most prominent signal that the female body is entering it. There is a major difference, however, between menopause and menarche. While the first menstrual period is unmistakable, the last menstrual period is only certain in retrospect—enough time has to elapse before one can be sure.

Understanding menopause as a transitional process involves understanding
the hormonal changes that accompany the transition, the symptoms associated
with the process, as well as the various aspects of life that influence the
experience along the way