Just as "pornography" is hard to define, so too is "yoga."
Some prefer to think of yoga as simply an exercise composed of challenging physical poses. It is far more than that if one considers its origins and the original contexts within which it was practiced and developed (see my "Thinking About Yoga? Make Sure to Think the Right Thoughts!"). And, like pornography, while there may be a general sense of what it is ("I know it when I see it"), one could never "succeed in intelligibly" defining it.
Though the word yoga itself derives from the Sanskrit verb युज्, yuj ("to yoke, bind or unite"), one also has to be careful when translating from Sanskrit to English. According to scholar Gerry Larson, the term yoga "is seldom used in the sense of 'yoke,' 'join' or 'union' as is sometimes claimed in popular accounts of Yoga." Larson shows that the term yoga more accurately and more generally refers to any sort of disciplined practice, and in this specific context the term is used as "concentration" or "disciplined meditation." So, while many could say that they are yoga practitioners (yogis), each could be using the term differently and referring to different activities, cognitive, physical or otherwise.
One could be a yogi of Organic chemistry, another of Orgasmic chemistry.
One could be a yogi of photography, still another of pornography.
If one can use the term to characterize any sort of disciplined practice then the term is just too broad and, perhaps, not that useful. And if one were arguing a legal case concerning yoga, one would likely be cherry picking the definition or characterization that suits one's position (as did I for the "Thinking About Yoga?" piece).
And don't forget that when worrying about whether yoga is a religion, one needs also to define "religion." Like "yoga" and "pornography," there may be a general sense of what "religion" is ("I know it when I see it"), but one may never "succeed in intelligibly" defining it!