Being blind means your eyesight is at or below 10%. This is measured using an eye-chart, where 10% is the top line and below is not being able to read the top line. When you can't do that they have big letters that they test you on. When I was temporally blind I could'nt even see there was a letter there with my left eye, let alone what it said, but it has recovered to about 80% vision, whereas my right eye has a big grey cloud in the middle of it and I can just about read the second line so I am classed as partially sighted in this one with 20% vision. In practical terms, I can only read very big characters with my right eye and can't use it for normal reading or work out much of what I'm looking at.
Therefore, blindness can be from see nothing to being able to see light and colours but very little detail. Most normally sighted people don't realise this, including me, before I had these problems.