Martin Luther King Jr. envisaged a time when black and white Americans would no longer be segregated by race. To envisage is to imagine something that does not yet exist.
The words envisage and envision share the same Latin root meaning to see. Both words are very similar in meaning but do have slight shades of difference. If you envision something happening, you can see it happening in your mind. I'm sure you can envision yourself graduating from high school! If you envisage something, you can imagine it, but not necessarily see it. You can envisage world peace, but it may be harder to envision it.