1.Definition: (informal) very; used informally as an intensifier
"That is one fine dog"
2.Definition: being a single entity made by combining separate components
"Three chemicals combining into one solution"
3.Definition: eminent beyond or above comparison
"She's one girl in a million", "The one and only Muhammad Ali"
4.Definition: having the indivisible character of a unit
"Spoke with one voice"
5.Definition: indefinite in time or position
"He will come one day", "One place or another"
6.Definition: of the same kind or quality
"Two animals of one species"