As the title says. If you play a note simultaneously on two strings in the same octave, is it still considered a double stop?
The word "stop" refers to where the string is stopped, meaning where it ends. So, first think of the nut as the stop for an open string. When you finger the strings on the neck you are making a new stop. And, of course, double stop just means you are doing that on two strings.
It doesn't matter what the interval is. The point is how many strings you are changing by "stopping" them with your fingers.