You know, that's an interesting question you ask about the meaning of "toning". It isn't a specific physiological term, but it is one that is used over and over again in the real world. As a physiologist, I am stumped by its meaning. In my mind, you can train a muscle to become stronger through an increase in muscle contractile protein or you can train a muscle to become more aerobic and fatigue resistant by increasing mitochondrial protein. Whether this is "toning" or not beats me.
Sorry I babbled but this is a question that I have also asked. I think in most people's minds, toning means making the muscle more firm and lithe, but not bulky.