The Oxford Companion to Beer definition of
Apparent Extract is a direct measurement of the dissolved solids in brewers wort, gauged according to specific gravity. See specific gravity. A substantial portion of the dissolved solids will be removed from the wort during fermentation by the action of yeast, so the progress of the fermentation may be monitored by measuring the disappearance of these solids. However, the measuring method compares the weight of the dissolved solids with that of water. As the fermentation progresses the solute is no longer pure water but is a mixture of water and alcohol.
Alcohol has a specific gravity substantially lower than water. This means the “true” or “real” extract is greater than that measured directly, hence the term “apparent extract.”
See also extracts.