Click-through rate is a ratio having to do with Internet advertising. It refers to the ratio between the number of times an ad was clicked to the number of times it appeared on a website (impressions).

This is expressed in a percentage. If there are five hundred impressions of a single ad, for instance, and one hundred people clicked on it, then the CTR would be twenty percent.

The expression of this figure in percentage is more reliable than simply looking at the number of people who clicked on an advertisement over a period of time, or on a monthly basis. This is because site traffic can vary as time passes.
The only way of finding out if a strategy applied to the advertisement is effective is if the rate (and not necessarily the raw number of clicks) increases. This data is used to determine how successful an online advertising campaign is.

CTR refers to the number of times an ad was clicked on and does not give information on how many individuals clicked on the ad. If a person should click on an ad several times, the CTR increases and makes the data unreliable for advertisers.