Click-through rate — the percentage of impressions that result in a click. On YouTube, the percent who clicked after seeing the thumbnail.
CTR measures clicks divided by impressions. On YouTube, the typical CTR range is 2–10%; anything above 6% is excellent, below 2% is poor. CTR is determined almost entirely by the thumbnail and title — the actual video content has no impact on whether the click happens (it happens before the play starts).
CTR matters because YouTube uses it as a leading indicator. High CTR signals "viewers want this" and earns more impressions. The compounding effect: a 10% CTR video gets 5x more impressions than a 2% CTR video at the same starting point, and that gap widens over time.
On other platforms: email open rate is CTR's equivalent for inbox (industry baseline 20–30%, good >40%). On Reels/TikTok, CTR translates roughly to three-second view rate.