When a post only features one character.
For tagging purposes, tentacles only count as a character if they are attached to a visible creature (octopus, etc.). That is, a picture of just tentacles would actually be tagged with zero_pictured.
Note that each image in multi-image posts is tagged separately, which means that it's possible (for instance) to have both solo and duo tags on the same post.
The following tags are aliased to this tag: alone