Here's my (non professional take), from reading and listening to Jill Cook and Steven Low:
TL;DR: Tendonitis/tendinosis are obsolete terms. If someone uses them, they are probably 10 or more years behind the times. The current one is tendinopathy. Tendinopathies provoke little or no inflammation, and we still have a long way to understand them. In particular, every person's tendons suffer disrepair and tears during their lifetime, but only a few feel pain --which can be present with little or no damage.
Steven Low (Overcoming Gravity, Bad Posture, and soon, Tendonitis)
Connective tissue has less blood supplies than muscles, so it can't keep up if there's overuse.
Tendinopathies provoke little or no inflammation. There's not really such a thing as tendinitis.
An overloaded tendon goes into reactive tendinopathy: pain and microscopic degenerative damage that can be fully healed.
If stress continues, normal regeneration processes can't keep up and tendon disrepair occurs, where collagen fibres are deposited out of alignment, leading to reactive on degenerative tendinopathy. An MRI can show small damage.
Further damage produces a degenerative tendinopathy: the tendon fibers regenerate haphazardly which can't be reversed. Again, no inflammation, but large scale disruptions on tendon tissue, and the possibility of tearing under heavy loads.
Jill Cook et all:
Current trends in tendinopathy management
Tendinopathy (pain and dysfunction in a tendon) is a prevalent
clinical musculoskeletal presentation across the age spectrum, mostly
in active and sporting people. Excess load above the tendon's usual
capacity is the primary cause of clinical presentation. The propensity
towards chronicity and the extended times for recovery and optimal
function and the challenge of managing tendinopathy in a sporting
competition season make this a difficult condition to treat.
Tendinopathy is a heterogeneous condition in terms of its pathology
and clinical presentation. Despite ongoing research, there is no
consensus on tendon pathoetiology and the complex relationship between
tendon pathology, pain and function is incompletely understood. The
diagnosis of tendinopathy is primarily clinical, with imaging only
useful in special circumstances. There has been a surge of
tendinopathy treatments, most of which are poorly supported and
warrant further exploration. The evidence supports a slowly
progressive loading program, rather than complete rest, with other
treatment modalities used as adjuncts mainly targeted at achieving
Conference video: Jill Cook - Current concepts in tendinopathy rehabilitation