In-continuity in The DCAU
While there have been several animated series based upon DC Comics characters over the decades, what is commonly accepted as the "DC animated universe" refers to the stable of shows and films that spin off from Batman: The Animated Series, the original show in this universe. However, the following series, Superman: The Animated Series showed the first signs of superheroes crossing over when the title character had encounters with heroes such as The Flash and Green Lantern. Older shows such as Super Friends and newer shows such as The Batman and Legion of Super Heroes are not part of this continuity. Other productions, like the 2006 direct-to-video film Superman: Brainiac Attacks, are not to be considered part of the DCAU, despite utilizing similar character designs and much of the same voice cast as previous DCAU series.
Read more about this topic: DC Animated Universe