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The year 1842 saw the brewing of the world's first lager in Pilsen (now Plzeň, Czech Republic). Lager is a lower-fermentation beer, which means that the yeast, once done, sinks to the bottom of the vat rather than float to the surface. Pilsner-type lager caught Europe by the storm, which had the unexpected side-effect of bakers no longer having access to a source of fresh yeast, as there was nothing to collect from the surface of the lager brew. And fresh yeast was indispensable. In the case of sourdough, it's common to leave some of it to kick-start the next batch, with the resulting flavour only getting better every time. This approach doesn't work with yeast, though, as old yeast gives bread a rather unpleasant, stale aroma.
A method, therefore, was needed to somehow rejuvenate this old yeast. And this is why pre-ferments were invented. The idea is to effect preliminary fermentation and let new generations of yeast cells to bud out from the old ones. Only then comes the second phase of fermentation whose purpose is to rise the dough with the young population of yeast. Pre-ferments come in several kinds, such as the Italian ''biga'' (used in production of ciabattas) or the English sponge (used for in making hamburger buns). But what we're interested in is the highly-hydrated pre-ferment, in which flour and water are mixed in equal proportion by weight, producing a loose starter similar in its consistency to pancake batter. This method is mostly used in preparing the dough for baguettes and other typically French rolls. And this is the method which is known by the word "poolish". Phew!
<mobileonly>[[File:Poolish.jpg|thumb|upright|left|Poolish – above, freshly mixed; below, risen]]</mobileonly>
