The process of a visitor clicking on a web page/app and going to the next page/sceen.
Official WAA definition: Number of times a link was clicked by a visitor.
Click-throughs are typically associated with advertising activities, whether external or internal to the site. Note that click-throughs measured on the sending side (as reported by your ad server, for example) and on the receiving side (as reported by your web analytics tool) often do not match. Minor discrepancies are normal, but large discrepancies may require investigation.