| aspen | (noun) any of several trees of the genus Populus having leaves on flattened stalks so that they flutter in the lightest wind | - |
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| ballpen | (noun) a pen that has a small metal ball as the point of transfer of ink to paper | Synonyms: ballpoint, ballpoint pen, Biro |
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| bullpen | (noun) a large cell where prisoners (people awaiting trial or sentence or refugees or illegal immigrants) are confined together temporarily | Synonyms: detention cell, detention centre |
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| (noun) a place on a baseball field where relief pitchers can warm up during a game | - |
| open | (noun) information that has become public | Synonyms: surface |
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| (noun) a tournament in which both professionals and amateurs may play | - |
| (noun) a clear or unobstructed space or expanse of land or water | Synonyms: clear |
| (noun) where the air is unconfined | Synonyms: open air, out-of-doors, outdoors |
| pen | (noun) female swan | - |
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| (noun) a writing implement with a point from which ink flows | - |
| (noun) an enclosure for confining livestock | - |
| (noun) a correctional institution for those convicted of major crimes | Synonyms: penitentiary |
| (noun) a portable enclosure in which babies may be left to play | Synonyms: playpen |
| pigpen | (noun) a pen for swine | Synonyms: pigsty, sty |
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| playpen | (noun) a portable enclosure in which babies may be left to play | Synonyms: pen |
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