From Longman Dictionary of Contemporary Englishsliced breadˌsliced ˈbread noun [uncountable] 1 
DFbread that is sold already cut into slices2 → the best/greatest thing since sliced bread
DFbread that is sold already cut into slices2 → the best/greatest thing since sliced breadExamples from the Corpus
sliced bread• He wrote more shampoo, hairspray, deodorant and sliced bread commercials.• Now, I didn't get it because I was the greatest thing since sliced bread.• Nina brought me a plate of smoked salmon and some sliced bread.• Have thick crust pizza, extra potatoes, thick sliced bread or rolls with your meal.• On a smaller plate set before Frankie was a sandwich made with thickly sliced bread, best butter and strawberry jam.• White sliced bread versus wholemeal bread is a simple example of this change at work. 