A cream, lotion, or oil you rub on your skin so the sun cannot burn it. The name says the idea: it *blocks* the sun. Originally people used 'sunblock' for the strong, thick kind that sits on top of your skin and stops the light physically — the white paste you see on a lifeguard's nose — while 'sunscreen' meant the kind that soaks in and filters the light. Today most people, and most shops, use the two words for the same thing, and 'sunscreen' is the more common word. Note that no product really blocks all of the sun, so in many countries (America, Europe) companies are now not allowed to print the word 'sunblock' on the bottle; you will still hear people say it, but you will see 'sunscreen' or 'sun cream' on the label. In Britain 'sun cream' is the everyday word. It is an uncountable noun most of the time — 'put on some sunblock', not 'a sunblock' — though you can say 'a good sunblock' when you mean a particular product.