Yesterday Nielsen made its Cross-Platform Campaign Ratings commercially available, following trials by GroupM, ESPN, Facebook, Hulu and Unilever. Cross-Platform provides, for the first time, "unduplicated and incremental reach, frequency and GRP measures for TV and Internet advertising." Cross-Platform brings together Nielsen's Online Campaign Ratings (OCR) with its longstanding national TV panel.

The Cross-Platform launch follows Nielsen's announcement two weeks ago that 15 leading video and digital ad platforms have integrated OCR, plus last week's news that the CW Network will use OCR to measure its online viewership and offer demographic guarantees for online advertisers for the 2012-2013 TV season. As I explained last week, Cross-Platform is so critical to online video advertising because it helps align the ecosystem with media buying expectations of the multi-billion dollar TV advertising industry.

While online video advertising has been steadily growing, and this year should account for about $3.2 billion according to eMarketer, it's still tiny compared to TV. A key constraint has been content providers' inability to represent their inventory or evaluate campaign results in the context of traditional TV spending.

That has led content providers to rely more on typical online metrics like impressions and click-throughs, while not being able to fully synch results with concurrent TV campaigns. As a result, agencies and brands haven't been able to optimize the planning of multi-platform campaigns, holistically understand the results or gain TV-style make-goods if campaigns fall short. With last spring's elaborate Digital Content NewFronts seeking to position major online video providers as alternatives to TV, these deficiencies have become more pressing. Cross-Platform ratings helps resolve this, which should in turn contribute to higher online video ad spending.

Next up, as Amit Seth, Nielsen's EVP of Global Media Products, told me yesterday, is to incorporate measurement of social into Cross-Platform and to expand its use internationally. To be sure, not all brands or agencies are sophisticated enough yet to effectively use online video advertising, much less Cross-Platform ratings. But as audiences continue shifting their behaviors, brands and agencies will be under growing pressure to advertise where the eyeballs are moving. That makes Cross-Platform an important milestone for online video advertising, which is in turn an important element in the larger theme of how advertising itself must change to stay relevant for increasingly empowered consumers.