The "Kegel" was first described in 1948 by Arnold Kegel, an Assistant Professor of Gynecology at University of Southern California. In his quest to treat "genital relaxation," he realized that the strength of the pelvic floor muscles was an important component. He developed a "perineometer" a handy dandy tool that to measures the pelvic floor strength, and coined the term "Kegel" to describe the voluntary contraction of the pelvic floor muscles.
So really, a Kegel is one man's attempt to describe the "concentric" (shortening) action of the pelvic floor muscles to achieve the goal of lifting and supporting the pelvic organs. He certainly blazed some important trails for the description and measurement of muscle strength, and his discoveries laid a foundation for the successful conservative treatment of many pelvic conditions.
However, Kegel is not the last word on pelvic floor muscle activity! Ideas have evolved and the body of evidence-based practice has deepened and widened significantly since the mid-century.
A Kegel is no longer the only tool we have in our toolbox, in fact it's practically just the tip of the iceberg when it comes to pelvic floor lengthening and strengthening. In fact, while "kegeling" can be really useful for some pelvic floor dysfunction issues, we also know that this type of exercise may actually not be helpful for many types of pelvic floor dysfunction.
A physical therapist trained in the specific evaluation of the pelvic floor muscles will not only evaluate how the muscle can squeeze and hold (a Kegel), but how well it can relax. A Kegel is only half the story, if even that much! What's more, if you would benefit from Kegels, you may need to be doing them in a different position than in sitting or standing! Depending on your strength and endurance, your PT may prescribe pelvic floor exercise that will be more efficient and give better results.
Kegel AH. "The nonsurgical treatment of genital relaxation; use of the perineometer as an aid in restoring anatomic and functional structure". Ann West Med Surg. 2 (5): 213–6.