It would be hard to improve on this description of Achilles tendinitis from the American Academy of Orthopedic Surgeons:
"Simply defined, tendinitis is inflammation of a tendon. Inflammation is the body's natural response to injury
or disease, and often causes swelling, pain, or irritation. There are two types of Achilles tendinitis, based upon which part of the tendon is inflamed.
1. Noninsertional Achilles tendinitis
Noninsertional Achilles Tendinitis
In noninsertional Achilles tendinitis, fibers in the middle portion of the tendon have begun to break down with tiny tears
), swell, and thicken.
Tendinitis of the middle portion of the tendon more commonly affects younger, active people."
"2. Insertional Achilles Tendinitis
Insertional Achilles tendinitis involves the lower portion of the heel, where the tendon attaches (inserts) to the heel bone."
"In both noninsertional and insertional Achilles tendinitis, damaged tendon fibers may also calcify (harden). Bone spurs (extra bone growth) often form with insertional Achilles tendinitis."
In most cases, nonsurgical treatment options will provide pain relief, although it may take a few months for symptoms to completely subside. Even with early treatment, the pain may last longer than 3 months. If you have had pain for several months before seeking treatment, it may take 6 months before treatment methods take effect.
The first step in reducing pain is to decrease or even stop the activities that make the pain worse. If you regularly do high-impact exercises (such as running), switching to low-impact activities will put less stress on the Achilles tendon. Cross-training activities such as biking, elliptical exercise, and swimming are low-impact options to help you stay active."
"If you have experienced a sudden "pop" in the back of your calf or heel, you may have ruptured (torn) your Achilles tendon. See your doctor immediately if you think you may have torn your tendon."
There is more information at the above site.
I think they avoid the term tendonosis so as not to confuse by introducing yet another term.
But basically tendonosis would be the end stage, chronic, degenerated tendon phase that is preceded by active tendonitis symptoms.
I think if the pain is really bothering you, holding off even on the "non-impact" cross training mentioned above would be wise until you see the doctor.
One of the things that is apparent, is that orthopods see extreme cases - usually in those that "push through the pain" and don't rest.