Final answer:
Click-through refers to the rate at which users click on an advertisement or a link, measured by the click-through rate (CTR), which is the number of clicks divided by impressions.
Step-by-step explanation:
Click-through refers to option 1) The rate at which users click on an advertisement or a link. This term is commonly used in online advertising to measure the success of an online advertising campaign. The click-through rate (CTR) is a key performance indicator that divides the number of users who click on an ad by the number of times the ad is shown (impressions), expressed as a percentage. A higher CTR indicates that the advertisement is effective at capturing users' interest. An example of this could be, if an advertisement is shown 100 times and receives 10 clicks, then the CTR would be 10%.