Definition of number one
(Entry 1 of 2)
1
: one's own interests or welfare : oneself
looking out for number one
—often written No. 1
2
: one that is first in rank, importance, or influence
—often written No. 1
number one
adjectiveDefinition of number one (Entry 2 of 2)
1
: first in rank, importance, or influence : foremost
cancer is the country's number one killer
—often written No. 1
2
: of highest or of high quality
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Merriam-Webster unabridged
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Time Traveler for number one
The first known use of number one was circa 1705
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