Universe
The list of stocks a test is allowed to choose from.
Before a strategy picks anything, someone decides what it may pick from. That list is the universe: all US stocks, only the five hundred largest, only companies above some size, only the ones with borrow data. Every rule is then tested inside that fence and nowhere else.
The fence changes the answer. An effect found across thousands of small, thinly traded companies can vanish when the same rule is run on the largest few hundred, because the stocks that carried it are no longer allowed in. The idea did not change; the universe did. That is why every log entry names its universe next to its result, and why a promise measured on one list is not a promise about another.
The universe also has to be honest about time. A list built from today's survivors, applied to the past, quietly excludes every company that failed along the way. The logbook rebuilds its lists date by date, including the delisted, and discounts any result that could not.
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Where this word comes up
First needed in the log entry H-059 · Testing the shorting premium: what a rebalancing bug taught us.
From the same entry: Borrow fee, Crowded shorts, Decile, Delisted, Long-short portfolio, Rebalance, Recall, Short interest percentage, Short selling, Squeeze, t statistic.