Short definition
CTR is the percentage of users who click a link out of those who saw it, used to evaluate ad creative, search listings, and email subject lines.
In depth
CTR is a direct test of how compelling a piece of content is at the moment of decision. In SEO, CTR per query is a leading indicator that title tags and meta descriptions are working. In paid media, low CTR is the first sign of weak creative or wrong audience. Improving CTR usually pays compounding returns: more clicks per impression means lower CPC in PPC and stronger ranking signals in SEO.
Example
Rewriting a service page's title from generic to a benefit-led headline lifts SERP CTR from 3.2% to 7.5%, doubling clicks without rank change.