Some legal opinions have been proferred that it could be reversible, even though if you read the text of A50 there's no mention of such a mechanism.
Personally I would have thought that was something you would want to exclude - article 50 could be used as a tool to extract concessions "We're leavingg unless...".
There's apparently a court case in Ireland to try to establish this.
Also there's the possibility it could be reversed with unanimous consent from the other 27 members because, when it comes down to it, the members make the rules.
It's entirely possible that the UK could be made to leave and reapply though, which would make for many more years of turmoil.
I don't personally think this is a very useful distinction. By this definition, a soft Brexit is what for example the Lib Dems are advocating at the moment. However, I think it is also highly unlikely to happen, because to a first approximation it is leaving-in-name-only and thus probably an untenable position for the government to adopt politically. The entire spectrum of other possibilities, from retaining relatively close ties in some respects up to completely walking away with no deal at all, then fall under the single banner of hard Brexit, but treating such widely different possible outcomes as equivalent doesn't seem to be very useful (unless for political reasons you want to try to equate them anyway, I suppose).
"Tea?"
"OK, go on."
"There's no kettle so I'm burning the house down to heat water."
"Now wait a…"
"Shut up, I've a clear mandate for tea."
https://twitter.com/louisejjohnson/status/829617043582369792
I suppose to be balanced we should also acknowledge the other side's similarly daft argument that they accept that people voted for tea and yet propose a scheme in which you can't buy any of tea bags, tea leaves, a kettle, an urn, or an instant hot water tap, though it's possible that milk or lemon will still be negotiable. :-)
(I'm not in the UK, apologies if this is a common question.)
[edit] Thanks for the answers!