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."