A unit, or a symbol ( 1 or I) representing it.
A single thing or person; any person indefinitely.
Single; indefinite; this; that; the same.
But I want one. – The Eustace Diamonds by Anthony Trollope
But- one thing he could do. – The Sea Bride by Ben Ames Williams
No one else could. – Norston's Rest by Ann S. Stephens