Tendinitis refers to an inflammatory process of the tendon. This occurs where the tendon inserts into the bone. Tendinosis is a degenerative process. With tendinosis, it's likely that a repair process that included early inflammatory changes took place.
But the events were disrupted. When tissue is examined from the area, there are no signs of inflammatory cells. Instead, fibroblasts and immature blood cells are present. The tendon becomes fragmented and frayed.
Tendinosis describes a chronic, degenerative process. Tendinitis refers to an inflammatory lesion. The terms tendinitis and epicondylitis are no longer used to describe chronic tennis elbow. With improved understanding of the underlying problem, the more accurate term (tendinosis) is used instead.