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1. the smallest whole number and the first cardinal number; unity
2. a numeral (1, I, i, etc.) representing this number
3. Music the numeral 1 used as the lower figure in a time signature to indicate that the beat is measured in semibreves
4. the one (in Neo-Platonic philosophy) the ultimate being
5. the Holy One or the One above God
6. the Evil One Satan; the devil