tl;dr: No, *Gotham* is separate, as is *iZombie*, but the rest of the DC shows are all effectively in the same universe.

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In the 2014-2015 and 2015--2016 TV seasons, there's **a ton** of DC shows on the air:

* *Arrow*
* *The Flash*
* *Constantine*
* *Gotham*
* *iZombie*
* *Supergirl*
* *Legends of Tomorrow*
* *Lucifer*

The only explicitly shared universe in that group is the "Arrowverse" which includes the three shows aired on CW and produced by Greg Berlanti, Andrew Kreisberg and Marc Guggenheim: *Arrow*, *The Flash*, and *Legends of Tomorrow*.  

At [Comic Con 2014](http://io9.com/ras-al-ghul-comes-to-arrow-and-other-dc-tv-news-from-co-1611625466) it was stated that there's no shared universe among the others, as of yet:

> While we know that *Arrow* and *The Flash* will crossover a lot, it was deemed a little difficult to cross over the non-CW shows, since *Gotham*'s time period is a little uncertain. *Gotham* executive producer Bruno Heller said, "Never say never" but pointed out "If those two [*Gotham* and *Constantine*] worlds coincided, then it would work. But I'm not really sure of the chronology. Maybe *Constantine* is only five when *Gotham* takes place."

So far, *iZombie*, *Lucifer* and *Gotham* have remained separate, and their setting and tone are diverse enough that they'll probably remain that way. However, after *Constantine* was cancelled by NBC, Berlanti's team got Matt Ryan to reprise his lead role on S4E05 of *Arrow*, meaning that *Constantine* retroactively joined the Arrowverse. In addition, in S01E18 of *Supergirl* (also produced by Berlanti's company), Grant Gustin appeared as Barry Allen, also bringing *Supergirl* into the Arrowverse continuity.