What is an impression?

Served, rendered, viewable

Three different events get called an impression, and the difference between them is where most measurement disputes originate.

Served means the ad server returned a creative. It may never have rendered.

Rendered means it loaded in the page or app.

Viewable means it actually entered the user's viewport for long enough to be seen — the standard definitions require a proportion of pixels for a minimum duration.

The gap between served and viewable is routinely 30–50% on display inventory, which is why buying on served impressions and judging on outcomes produces such disappointing arithmetic.

Why counts never match

• Different counting points. The publisher counts on return; the advertiser counts on load.
• Latency and abandonment between those two moments.
• Blockers, which prevent rendering after serving.
• Filtering. Verification vendors discount invalid traffic that the raw count included.
• Timezone and period boundaries, which shift volume between days rather than changing it.

What replaces it on AI surfaces

The impression assumes a rectangle in a viewport. Inside a generated answer there is no rectangle, and 'was it visible' is not the useful question — the useful question is whether the brand was part of what the user read.

Answer insertion is the emerging unit: the brand was named, cited or recommended within the response. It is countable for paid placement and has to be sampled for organic presence, which is what AI share of voice measures.

It is also worth substantially more than an impression, because it arrives with the assistant's implicit endorsement at the moment of decision rather than beside content the user was reading for another reason.