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I view it as a catch all term for all the small differences in how a lens represents the final image.
This includes bokeh, color accuracy, sharpness, macro and micro contrast, smoothness of transition from in-focus areas to out of focus areas. Smoothness and color of skin tones, etc.
Each lens will have subtle differences in how it 'renders' each of the above things...put together, and you sort of get the signature of how the lens draws, if you will. Sometimes technical flaws can actually make the lens more appealing to some people, and it's completely subjective. As I've used more and more lenses and such, I find that a very clinical rendering (things are sharp, medium contrast, no major flaws, unexceptional bokeh and quick transition from in focus to out of focus), while technically 'good' can sometimes be very boring. Others may completely disagree with those things.
So, when someone says they prefer the rendering from one lens vs another, they are just saying that they like how those image quality elements come together in that lens. For instance, the Sigma 50 f/1.4 and Canon 50 f/1.4 are similar in sharpness, the Canon has more accurate color (Sigma is warmer), and the Sigma has smoother bokeh. However, put all the things together, and I personally find the Sigma 50 to have a much nicer rendering for what I shoot. It's got that beautiful transition from in focus to out of focus..nothing is harsh, yet there's still nice crisp resolution.
Of my current gear the same could be said between the panasonic 20mm f/1.7 and the Leica 25mm f/1.4. Both are very sharp, but the 20mm is generally boring with busier bokeh, while the 25/1.4 has smoother bokeh and a presence that is hard to describe, but you know it when you see it.