Is it a person's outward appearance? Is it a person's core values and morals? Facial symmetry? Personality? Make-up? A toned body? Mental capacity? A great smile?
Here is dictionary.com's definition of the word 'pretty':
adjective, -ti·er, -ti·est, noun, plural -ties, adverb, verb, -tied, -ty·ing.
pleasing or attractive to the eye, as by delicacy orgracefulness: a pretty face.
(of things, places, etc.) pleasing to the eye, esp. withoutgrandeur.
pleasing to the ear: a pretty tune.
pleasing to the mind or aesthetic taste: He writes prettylittle stories.
(often used ironically) fine; grand: This is a pretty mess!
Informal . considerable; fairly great: This accident will costhim a pretty sum.
Archaic or Scot. . brave; hardy.
Usually, pretties. pretty ornaments, clothes, etc.
a pretty person: Sit down, my pretty.
fairly or moderately: Her work was pretty good.
quite; very: The wind blew pretty hard.
Informal . prettily.