Solo idea generation consists of generating ideas all by yourself. It's a practice that is seen as not very innovative by many, but it's still a setting that works pretty well.
Solo idea generation works well especially at the start of an ideation process where you don't want people to get biased to rapidly by the ideas of others and get stuck there.
In fact, in the idea generation workshops I lead, I always start with a few rounds of solo ideation. When I feel that people have emptied their personal brain, I'll then bring people together so that they can combine them to come up with new stuff.
Basics of Ideation for Innovation (Alpha version)
A future course teaching ideation theory and methods.