Any sauce that is deglazed and boiled down, or a dish with a sauce made that way.
ristretto di aceto balsamico: a sauce made with vinegar, sugar and spices, reduced and added to a dish at the last minute
brodo ristretto can mean a consommé, or any reduced sauce.
ristretto di pomodoro: a thick tomato sauce
A caffé ristretto is an espresso with slightly less water, said to bring out the aroma.