So... what is yoga?
Its root in the word 'Yuj', meaning either to bind, harness, connect or even concentrate is often quoted by budding yoga practitioners, with the goal of union on both an inter and extra-personal level offered as the follow up and the suggestion of something deeper that remains unclarified.
Although, perhaps most of all the word Yoga is synonymous with the practice of postures and (sometimes) breathwork. Indeed, it is the centrality of asana that is the defining aspect of 'modern' yoga. We unite breath and movement, or body and mind through postural practice, and indeed, we do see a gradual progression historically (more later) from the esoteric and ritualised practice of Vedically prescribed Yoga, to the Vedantic period that represented many of those rituals internally, through the Tantric and Hatha periods that increasingly divinised and utilised the body, to Colonial India and the work of Krishnamacharya and his famous students that are often cited as the seed of the myriad forms of physically oriented yoga we now see.
We see a shift also in that time from solitary, often male dominated and highly religious practice, to yoga that takes part in large groups, often female dominated and with a focus on therapeutic outlets influenced by modern science.
In her 2005 book 'A History of Modern Yoga' Elizabeth De Michelis proposed the term 'Modern Postural Yoga' as part of a broader framework to delineate the type of Yoga we so dominantly see. One of her students Mark Singleton, who ruffled many feathers with the publication of his book Yoga Body: The Origins of Modern Postural Practice in 2010, suggested the term 'Transnational Anglophone Yoga', although neither are widely accepted in academic, or practising populations.
The parameters of the terms modern versus pre-modern, and perhaps even eventually post-modern are tricky to place. Whilst the term pre-modern, often associated with the Classical Yoga Period, is seen as something pure, authentic and untainted, and perhaps even unobtainable for many 'modern' practitioners.
Furthermore, Yoga's place within Hindu religion and culture creates fierce debate, to the point famously even of court intervention (here's a little summary).
In his lectures, Christopher Wallis makes the interesting point that throughout the history of Yoga there has always been great interdisciplinary debate, discourse and disagreement about every aspect of practice, although, almost always an agreement of the purpose of Yoga. Whereas nowadays, there seems to be great agreement about what constitutes a yoga practice... but no-one can agree on what the 'true' outcome of successful practice is!