Well, this term essentially refers to the type of filtration used to prepare a whisky for bottling, where, as the name states, the whisky is chilled to a low temperature, usually around 0 degrees C, or lower, and forced through one or multiple metal or paper filters, under pressure. What this process does is remove naturally occurring particles from the whisky, namely fatty acids, proteins and esters, and sediment. But why? So the whisky does not go cloudy/hazy when it is chilled, and/or diluted to below 46% alcohol by volume (ABV%),and this is seen as undesirable by some whisky drinkers (especially when they add a handful of ice cubes to their Johnnie Walker), so some distilleries take this extra step during the production of their whisky to prevent this happening. Yes, that's it. That's all it does. Shocking, I know.
Also known as 'scotch mist', this cloudiness occurs when the above mentioned particles bind themselves together as a result of a temperature drop. This does not generally occur in spirits consumed above 46% ABV because the higher alcohol level stops the particles binding together, but if you add enough water to your cask-strength whisky to reduce the amount of alcohol in the glass to below 46%, it can still go cloudy. Another purpose of chill filtration is to remove any sediment from the whisky before bottling, which occurs during distillation, and also during maturation (coming from the cask). But this can be removed from the whisky using simple barrier filtration, without altering the temperature of the liquid, which retains most of the natural oils and particles in the finished whisky.
The chill filtering process is pretty interesting scientifically, at least to this layman, as the (apparently) unwelcome particles are filtered out using adsorption, rather than absorption. Basically this means the particles are collected as solids on the surface of the filter medium, rather than being absorbed or dissolved by it. Another interesting fact is this process actually adds to the production costs of the whisky! So come on Scotch (and other) whisky industries, why not save some money?
So how does all this affect your whisky (or other spirit, for that matter)? This is bit of contentious topic, as some parties (let's call them the 'yes' camp for added drama), both distilleries and consumers, claim chill filtration has no effect on the finished product, other than preventing the 'scotch mist' from occurring. On the other hand, those opposed to chill filtration in whisky (the 'no' camp), which I belong to, believe that the removal of these particles dampens, or more accurately strips, the natural mouth-feel and texture, and even the flavour & finish of the whisky. There have even been blind tastings where some experts cannot tell the difference between all tasted samples of whisky which has been chill filtered, and all samples of whisky which has not. But in this whisky enthusiast's opinion, chill filtration is evil!
Thanks to dramming.com for the image. Guess which sample has been chill filtered!
There are not many examples of the same whisky being offered in both non-chill filtered and chill filtered forms, but one way where this is possible is to look for an independent bottling (such as those from Duncan Taylor, Gordon & MacPhail, etc.) of a normally chill-filtered whisky, which states clearly that it is non-chill filtered.
Lastly, I should point out that a whisky being chill-filtered is not necessarily grounds to avoid it, it may still be a great whisky of excellent quality (looking at you, Lagavulin). The point is though, if it were non-chill filtered, it could be just that much better!