Viewability

Viewability measures whether an ad actually rendered on screen where a user had the chance to see it, rather than merely being served.

An impression counts as viewable when enough of the ad is on screen for long enough under the standard. Reddit reports this through third-party partners like Integral Ad Science and DoubleVerify, so you can check viewable rates independent of Reddit's own count.

Your campaign serves a million impressions, but the third-party viewable rate comes back at seventy percent, meaning three hundred thousand were served with no real chance to be seen.

Served and seen are different lines. People optimize to served impressions and ignore viewability; a cheap CPM with poor viewability can cost more per ad actually seen than a pricier, cleaner placement.

Pay for seen, not for served.