In Yoga Journal, I've found one description of yoga:
Classical yoga, however, is a holistic practice comprising eight
limbs—the physical postures being just one element of finding peace
The same author is aware that yoga can mean different things to different people:
Yoga in Western society often misrepresents the physical practice,
known as yogasana, as yoga itself.
It does not appear to me that yoga is meant as a form of treatment. Some doctors may suggest yoga as a relaxation method or exercise, though. Here are some recent systematic reviews of studies about yoga:
1) Yoga and anxiety (Depression and Anxiety, 2018):
There was inconclusive evidence for effects of yoga in anxiety
2) Effects of yoga on chronic neck pain (Clinical Rehabilitation, 2017):
Yoga has short-term effects on chronic neck pain, its related
disability, quality of life, and mood suggesting that yoga might be a
good treatment option.
3) Mind-body interventions for post-traumatic stress disorder (PTSD) (Journal of Clinical Psychology, 2018):
...the results for mindfulness, yoga, and relaxation are promising.
4) Noninvasive Nonpharmacological Treatment for Chronic Pain (Agency for Healthcare Research and Quality, 2018):
There is low evidence that yoga is associated with slight improvements of chronic low back pain.
5) Yoga for asthma (Cochrane, 2016):
We found moderate‐quality evidence that yoga probably leads to small
improvements in quality of life and symptoms in people with asthma.
6) Yoga for secondary prevention of coronary heart disease (Cochrane, 2015):
The effectiveness of yoga for secondary prevention in CHD remains
7) Yoga for epilepsy (Cochrane, 2017):
...no reliable conclusions can be drawn regarding the efficacy of yoga
as a treatment for uncontrolled epilepsy
8) Psychosocial interventions for recurrent abdominal pain in childhood (Cochrane, 2018):
There was no evidence for the effectiveness of yoga therapy or written
9) More recent Cochrane systematic reviews about yoga
In conclusion, yoga may help in relaxation and stretching, so it can relieve tension and thus reduce musculoskeletal pain in some cases. The evidence from studies does not suggest that yoga would have a direct healing effect on a tissue level, though. Some people who practice yoga experience less pain but yoga does not appear to treat the underlying disorders that cause pain.