This one is precisely locatable — most people can put a fingertip on the exact spot, on the tendon between the kneecap and the shin bone. That specificity is what distinguishes it from the more diffuse ache of runner’s knee.
Tendinopathy, not tendonitis
The older term was tendonitis, implying inflammation. Current understanding is that persistent tendon problems are more a matter of the tendon’s structure failing to keep up with the load placed on it. It matters, because it changes the treatment: anti-inflammatories and rest alone tend not to fix it.
The warm-up pattern
Characteristically it hurts at the start of a session, eases as you warm up, and then is worse afterwards and the following morning. People take the warm-up easing as a sign it is fine to continue. It is not — that pattern is the tendon telling you it is being overloaded.
What treatment involves
Soft tissue work through the quadriceps to reduce the pull on the tendon, treatment of the tendon itself, and work on the hip and ankle, since restriction at either end changes how the knee loads.
The essential part is graded loading. Tendons rebuild in response to progressive, controlled load — not to rest. A programme of heavy, slow resistance work is what changes tendon structure, and it takes months rather than weeks. Frustrating, but it works, and nothing faster does.