Nielsen has just announced a new form of television ratings. The Nielsen Twitter TV Rating will tell us all how socially engaging a given program is, based on Twitter users’ conversations and updates.

It’s one small step for Twitter, one giant leap for the legitimacy of second-screen data for marketing.

With a new agreement between the two entities, Nielsen will have data from Twitter that will allow it to create a “syndicated-standard metric around the reach of the TV conversation on Twitter.” Both parties say the new metric should be ready for the beginning of the new television season in the fall of 2013.

“Ultimately, we have one goal for this new metric: to make watching TV with Twitter even better for you, the TV fan,” Twitter media chief Chloe Sladden wrote today on the company blog.

For its part, Nielsen said in a statement that it considers Twitter the best source of real-time social data on TV shows. Most importantly, the vast majority of tweets are public rather than private, meaning Twitter data is easier to access than data from just about any other social source.

Twitter data won’t replace Nielsen’s current system of TV-tracking boxes; rather, it will act as a supplement in an industry (the ad industry) where more information is always better.