Click-through rate (CTR)
Clicks divided by impressions, shown as a percentage.
Click-through rate measures how often people who see a link or banner click it. If a banner is seen 10,000 times and clicked 30 times, its CTR is 0.3%.
For signature banners, impressions are estimates (see impression pixel), so CTR is an estimate too. It is most useful for comparing banners against each other.