Click Through Rate is the number of click that have happen after a user has seen a page

About

CTR is a web metrics that count the number of click that occurs after a user has seen a document.

Articles Related

Usage

It's an ad metric that counts the number of click by impression.
It's also an email metrics, it counts the number of click by email opened or send.

Tracking

The tracking and analytics occurs via UTM parameters

Documentation / Reference

wiki/Click-through_rate