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prothalamion
prothallus
prothesis
prothonotary
prothonotary warbler
prothoracic gland
prothrombin
protist
protium
proto-
Proto-Algonquian
Proto-Germanic
Proto-Indo-European
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proto-oncogene
Proto-Romance
Proto-Semitic
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protium
pro·ti·um [ prṓtee əm, prṓshee əm ]


noun 
Definition:
 
lightest isotope of hydrogen: the most common and lightest isotope of hydrogen, with atomic mass 1
[Mid-20th century. < Greek prōtos "first"]
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