Solo refers to material featuring a single performer rather than a pairing or group. In a gay context, that usually means one man on camera by himself, with no partner involved. The term says nothing about what else might be included, which is why it often sits alongside words like jerking, toys or cumshot in the same vocabulary. It is also worth separating solo from terms that describe a condition or a type of act between people. Bareback, for instance, is about condom use during partnered activity, so it belongs to a different kind of description entirely. Solo simply marks the number of people present.