I use this analogy to describe it. It's like a pitbull sinking its teeth into the neck of an animal it's attacking, it just won't let go, till it's beaten or killed. That's real tenacity! Try googleing the word tenacious, that would be a good start.
not giving up, being stubborn, stick-to-it-ness, making sure the job gets done. that is what it means. not sure if you wanted this or the actual def. which is the quality or property of being tenacious.