It would be *extremely* helpful if you tell us where you saw it. This is like asking what does the symbol $\partial$ means. It may have different meanings in different contexts. – 2012-08-03
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It's usually some generalization or specialization of the "union" sign, but it does greatly depend on context. – 2012-08-03
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My default interpretation is of disjoint union, thought it's also similar to the symbol often used coproducts. – 2012-08-03
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[Search results for `\bigsqcup` on math.SE](http://math.stackexchange.com/search?q=%5Cbigsqcup). – 2012-08-03
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@Andrew: disjoint union is a coproduct. I'd say it is almost certainly some coproduct. In what category, it would depend on the context. – 2012-08-03
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I simply meant that, though $\amalg$ seems to be the most common notation for general coproducts (not necessarily of sets), $\bigsqcup$ might have been used instead. – 2012-08-03
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I think it tells you where to put the staple. – 2012-08-03
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$\bigsqcup A$ is also used to denote (if existent) the smallest upper bound of a given subset $A \subseteq X$ of a partial ordering $(X, \le)$, but now that I think of it, this may also be viewed as a coproduct... Sorry. – 2014-06-17