"the" definite article.
"a" indefinite article.
So do you mean THIS one, this definite one that is there, or do you mean any one, any woman in a dress?
In this case, it seems you could use either, almost without change in meaning.
Do you see a woman in a dress? - Maybe the page has 3 such & you want her to find one.
Do you see the woman in the dress? - There must be one that you are referring to. So, maybe only one is there to be found. Otherwise you;d have to say something else to tell her which ONE you mean. Do you see THE woman in the dress, on the left, right, &c. ?
Do you see a cat on a bench? - potentially many cats on many benches - choose one, any one
Do you see the cat on the bench? - indicates a definite one, THAT one. So probably only one to be found.
Get me a plate from the cupboard. - any plate, which cupboard is understood from context in some way. same as get me any plate from the cupboard.
Get me the large plate form the cupboard. - There must be one definite plate that will be recognised as "the large plate".