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Definition of Ci
1. Noun. A unit of radioactivity equal to the amount of a radioactive isotope that decays at the rate of 37,000,000,000 disintegrations per second. 
2. Adjective. Being one more than one hundred.
Medical Definition of Ci
1. 
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1. Noun. A unit of radioactivity equal to the amount of a radioactive isotope that decays at the rate of 37,000,000,000 disintegrations per second. 
1.