An option in the "Goodies" menu of the first MacPaint applications. It let a user edit an image one pixel at a time. Any paint or image editing program that lets you zoom down to a single pixel is providing the same editing mode.
The Lasso's FatBits
This view zooms in on the lasso button, which is displayed in its normal size at the top right. At this depth, you can edit one pixel at a time.