Definition of "One" :
noun: a single person or thing
"He is the best one."
noun: the smallest whole number or a numeral representing this number
"He has the one but will need a two and three to go with it."
adjective: eminent beyond or above comparison
"She's one girl in a million."
adjective: indefinite in time or position
"He will come one day."
adjective: being a single entity made by combining separate components
"Three chemicals combining into one solution."
adjective: (informal) very; used informally as an intensifier
"That is one fine dog."
adjective: of the same kind or quality
"Two animals of one species."
adjective: used of a single unit or thing; not two or more
adjective: having the indivisible character of a unit
"Spoke with one voice."