Drawing has a number of definitions because it has a number of purposes. What most definitions have in common is observation, exploration, preparation, and visual evocation of the material world. They are judged successful by their qualities (e.g elegance, gravity, wit etc.), their inventiveness, truth, accuracy of depiction and insight. Also whether they are good preparation for work which is to follow, for example by formulating good compositions or spaces for paintings, or good indication of form for painting or sculpture. All this visually, for in the extent to which you need words to explain a drawing, to that extent it has failed.