Body is a measure of the density of the espresso. Sometimes called mouthfeel, it reflects the weight of the coffee solids that make it into the cup. The deepness and richness of an espresso's color is also a measure of its body.
We rated a full-bodied espresso higher than one that's thin, weak, or diluted. Therefore we often gave a shorter pour - or an espresso made with the same amount of coffee but less water - a higher rating. A shorter pour is more likely to possess its own natural sweetness, and a longer pour is more likely to taste bitter.
An espresso shot is called over-extracted if an excessive amount of water is used to draw the flavors out of a given amount of coffee. Not only does an over-extracted espresso taste weak and bitter, but also it generally contains more caffeine - which is mostly derived from the water-soluble elements of the ground coffee. Therefore, a properly made espresso is neither bitter nor more caffeinated than a typical cup of joe.