King Arthur's definition:
A preferment is a preparation of a portion of a bread dough that is made several hours or more in advance of mixing the final dough.
The subject of preferments is one that can cause immense confusion among bakers. The variety of terminology can bewilder even the most experienced among us. Words from foreign languages add their contribution to the complexity.
A preferment is a preparation of a portion of a bread dough that is made several hours or more in advance of mixing the final dough. The preferment can be of a stiff texture, it can be quite loose in texture, or it can simply be a piece of mixed bread dough. Some preferments contain salt, others do not. Some are generated with commercial yeast, some with naturally occurring wild yeasts. After discussing the specific attributes of a number of common preferments, we will list the benefits gained from their use.
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These terms all pertain to preferments; some are quite specific, some broad and general. The important thing to remember is that, just as daffodils, roses, and tulips all are specific plants that fall beneath the heading of “flowers,” in a similar way the above terms all are in the category of “preferments.” Let’s examine several of the terms listed in more detail.http://www.kingarthurflour.com/professional/preferments.html