The condition used to be called patellar 'tendinitis' or 'tendonitis'. The term '-itis' in medicine means 'inflammation' - a process of irritation, increased blood supply, and infiltration of the area with special inflammatory cells - but these features are not actually seen in patellar tendinitis. So there is a lot of medical literature suggesting that the condition no longer be called 'patellar tendinitis' but 'patellar tendinopathy' or 'patellar tendinosis' instead, implying a degenerative condition rather than an inflammatory one. For your research, you need to search using all of these keywords.
In this disorder the tendon develops a mushy ('mucoid') degeneration, which weakens it, causes local discomfort, and may predispose the tendon to rupture.