CTR (click-through rate)
CTR is clicks divided by impressions, expressed as a percentage. It measures how compelling an ad is to its audience in context, and is a useful creative and relevance diagnostic - but a high CTR with poor post-click conversion is curiosity, not performance.
In practice
CTR norms vary enormously by channel, format and intent level; a strong search CTR and a strong display CTR are different orders of magnitude.
Use CTR to compare creative and audiences within a like-for-like context, and to spot fatigue when it declines on a stable audience.