Where does Eleven's mind goes when she is in the sensory-deprivation tank? Is it alternate reality? Or is it parallel universe? Did she time travel or space travel?
It is called isolation tank or sensory-deprivation tank:
An isolation tank is a lightless, soundproof tank inside which subjects float in salt water at skin temperature.
That said, you can see that she is able to project her mind near a target in a different geographic area and listen to them talking (when Dr Brenner uses that to eavesdrop the Russians).
Using a sensory-deprivation tank and hooked to the wave machine, they use her to collect information from a Russian man...
She then flashes-back to finding the Russian, where she "casts Shadowwalk" - projects herself on the astral plane to locate people - on him, but finds the monster and it gives chase
Doesn't look like she can time travel - at least not in the first season.