Perhaps a little late in the day to be asking this. But despite my attempt to read every book written on yoga, I'm not sure I can give you a definitive answer. The closest I can get to one is;
Yoga is whatever you want it to be.
For Andrea Jain, Yoga is contextual. Modern yoga systems, like postural yoga bear little resemblance to the yoga systems that proceeded them. What modern yoga systems share with premodern ones is they are specific to their own social contexts.
Another yoga academic Georg Feuerstein tells us that yoga is a spiritual discipline which aims at liberation. All forms of spirituality share a strong interest in tapping into the highest human potential. Authentic yoga goes beyond the conventional levels of human activity, the all pervasive worry over food, shelter, companionship, material means, etc. It's a process of discovering our dharma and then removing the obstacles to living it.
A slightly lesser known yoga academic (me) suggests that yoga is a process of self awakening, a space to witness the actual experience of breath, body and mind, without trying to control the outcome. It's play and a chance to find out who you really are when you let go of analysis and critique. The principles of the self awakening yoga I teach help students explore sensation, and provide personal access codes to the wisdom in their body, (although I don't put that in my class description).
Yoga is contextual, a spiritual discipline and a journey of self awakening. It's also about self care and social justice. I can't seek to understand myself, feel at ease and wear a snazzy top with the slogan don't hate, meditate and leave it there.
There are a lot of things to feel angry about, to lose sleep over and too often the systemic problems are repackaged as personal. Here's an example; the reason I sometimes find it hard to relax is because I look at the news. If I just stopped looking at it, I'd be fine.
Well no; the rolling news thrives on panic, it's got people completely addicted to regular updates and the nature of headlines, means complex situations are reduced down to a single statistic. This robs people of context and sufficient information to make their own decisions. It's dangerous.
I want to be a part of a meaningful solution to suffering (that's my dharma), not a private, individualised salvation. For many reasons I choose not to follow the news, but that can't be where this story ends. In sharing what I'm up to and why, I'm letting you know there's another way. We can look after ourselves (individual liberation) as well as stop sponsoring any company, organisation or person that causes harm (global liberation).
To quote the writer, feminist and social activist Audre Lorde;
Caring for myself is not an act of self indulgence, it's self preservation, and that in itself is an act of political rebellion.
So yea... Yoga is whatever you want it to be, and I really hope it's more to you than a nice stretchy time. It might begin like that, but you allow it to flourish into a personal and global technology for change, a disciplined practice of liberation for all and... despite it's vast potential for political rebellion, it remains playful.