The main differentiator is programmatic fills otherwise vacant inventory with audience/intent/etc placements and is not purchased through the publisher directly. To get to the definition you have to go back before ad networks existed. Companies owned a website, and you went to the 'ad sales' section of the website and negotiated packages with the company. You'd get ads placed for a limited amount of time and/or impressions, and the purchase was similar to what you'd do for a Newspaper or Print ad today. Then came ad networks. Publishers could add them in to give eyeballs to remanent inventory. Say they had 1 million visitors in a day, but only sold 500k impressions directly. The ad networks pick up the other 500k for the highest bidder (and the publisher makes more money). The networks started directly placing buys themselves, but quickly realized they could automate it to optimize performance for both advertisers and publishers - and thus multi-site and programmatic advertising was born. An advertisers could now say "get me as many impressions as you can for under $3 CPM" and the networks could go out across tens of thousands of sites, enter auctions, and deliver. After that, layering on third-party data, intent, etc. was just the natural evolution of ad network buys. So to that point, you can have a buy on the same site and one can be 'programmatic' and one cannot. I can buy direct from CNN.com for 1 million impressions. That's not a programmatic buy. I have selected exactly where I want the ads to serve and can negotiate if I want homepage, politics, etc. placement. I can also buy through an ad network, and when CNN has remnant inventory (inventory that has not been purchased direct) it will serve up in the auction, allowing it to get auctioned and placed by those various networks to the highest bidder. I can set preferences, whitelist placements, etc. but I'm not guaranteed to serve unless remnant inventory becomes available on those placements AND I want win the auction to get placed. The first is not a programmatic buy. The second is. Google search is not a programmatic buy because you are buying on a specific site (Google) with specific terms/phrases/etc. that you are directly negotiating with Google (within the platform). Google display is a programmatic buy because it runs on an ad network that fills remnant inventory across millions of publishers, but you are having no direct negotiations with said publishers. Youtube, purchased through Google ads platform, is not a programmatic buy because you have selected the broader site and are instead running on channels within it. If you run a digital video campaign through Trade Desk or another vendor, and YouTube is one of the channels available in the targeting set along with a hundred other OTT or streaming app networks, then it could be a programmatic buy. Basically programmatic refers to specifically how the inventory is getting placed (direct vs. through a network). The audience targeting, intent, etc. are all part of programmatic platforms but don't inform the definition. Answer from OddProjectsCo on reddit.com