What it means

1. adjective being a single entity made by combining separate components
   “three chemicals combining into one solution”
2. adjective used of a single unit or thing
   “`ane' is Scottish”
3. noun a single person or thing
   “he is the best one”
4. 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”