In the same spirit as this question about aspect ratios...
A cold open is a narrative tactic where instead of showing the opening sequence at the beginning, the show jumps right into the story.
I've noticed that these days, most anime use cold opens. In fact, there are a number of anime, such as Symphogear (2012), Railgun S (2013), and Mekakucity Actors (2014), where the first episode is basically a super ultra great cold open: They put the opening sequence where the ending sequence usually is, and just omit the ending sequence entirely for the first episode.
However, in the past, this was not the case. For example, older anime such as Dragon Ball Z (1989) and Eureka 7 (2005) always put the opening right at the beginning.
So, when did the transition to cold opens happen?