How do different traditions define "intoxicant"? Since the mapping of Novel Psychoactive Substances (NPS) began about ten years ago, around 500 new subsatances  has been discovered. This is a somewhat different situation than what was the case 2500 years ago. 

Some NPS are not illegal, and even it they are defined as pshychoactive they're not intoxicating but more like caffein ("upper") or melatonin (for sleep) or valerian (calming). 

What defines - in the different traditions - whether taking an NPS is against the five precepts or not? Is it the intention with which you take the substance? Is it the effect? Neurobiological damage? Physical damage? Frequency? Does legal status come in to the picture? 

Is this something one should take directly with the teacher? Or something you can define according to your own conscience?
Let me first explain the original Pāli version of the fifth precept, according to what I learnt from prof. Richard Gombrich, who knows Pāli well and is a Theravāda scholar, but is not a Buddhist.


  Surāmerayamajjapamādaṭṭhānā veramaṇī sikkhāpadaṃ samādiyāmi.


The last three words are the same for all precepts and mean "I undertake the rule of abstaining from". The key word here is surāmerayamajjapamādaṭṭhānā. It is a compound that can be split as follows: surā-meraya-majja-pamāda-ṭṭhānā. The literal translation is surā-meraya-majja-carelessness-occasion. Surā, meraya and majja are names of three intoxicating drinks, but we cannot be sure what they exactly were. So if we look at it literally, the rule deals with three specific drinks, and whether and how it shoud be extended to other intoxicants is a matter of interpretation.

The use of pamāda (carelessness) is even more interesting. Since it is a part of a compound, the grammar rules of Pāli tell us very little about how it should be interpreted. So the literal reading of the fifth precept could be either


  I undertake the rule of abstaining from surā, meraya and majja, because they cause occasions for carelessness.


or


  I undertake the rule of abstaining from surā, meraya and majja, in so far as they cause occasions for carelessness.


Coming back to your question, there is no agreement among Theravāda Buddhists on which of the two readings should be used. There are quite a few who interpret it according to the latter reading and don't think that consumption of small amounts of alcohol is against the precept. But in any case, there is an agreement that surāmerayamajja should interpreted as alcoholic beverages in general. There is, however, a controversy on whether the precept is applicable to smoking.

To sum up, various Buddhist traditions have surely different views on that matter, but in any case, the word pamāda seems to be the key. It is what defines intoxicants, and - for traditions that allow some use of intoxicans - defines the limits of their use.