What is a unit of alcohol?

An image with 5 drinks, with the heading what does 1 unit of alcohol look like? The first drink from the left is a glass of cider, with the labels 218 ml and standard 4.5% cider. The next is a glass of white wine, with the labels 76 ml and standard 13% wine. The third is a glass of whisky, with the labels 25 ml and standard 40% whisky. The fourth is a glass of beer, with the labels 250 ml and standard 4% beer. The last is an alcopop bottle, with the labels 250 ml and standard 4% alcopop (275 ml).