1. An artifical passage for water, fitted with a valve or gate, as in a mill stream, for stopping or regulating the flow; also, a water gate of flood gate.
2. Hence, an opening or channel through which anything flows; a source of supply.
There was a reasonably good path now, mostly on the edge of the river, with a divergence here and there where a dike came, with a miniature windmill on it and a muddy sluice-gate.
Charles Dickens. Great Expectations (1861)
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wondering what sort of shape the Oxbow Dam was in, and if the sluice gates had been left open or shut.
Stephen King. The Stand (1990)
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The water gushed happily down the sluice under the idle water-wheel into the millpond.
Diana Gabaldon. Outlander (1991)
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All of a sudden the lightning let go a perfect sluice of white glare,