Stray (偶発的にやって来る異世界人 Gūhatsu-teki ni yattekuru i sekai-jin, lit. "Otherworlder transported by accident") or Stranded One or Accidental Otherworlder refer to those Otherworlders who came from Earth (or possibly other worlds) by sheer accident. There is no clear cause or pattern known for why people get transfer between worlds like that, but in general, it's considered a one-way street and the phenomenon is not known to happen into the opposite direction.
Among the three Transmigration types, it's a dangerous way to be transported in the initial stage, since there is no fixed target location. So it's possible to be suddenly transported into the middle of a territory that is hostile to humans, in a completely unprepared state, possibly even into a hazardous region that leads to almost instantaneous death upon arrival. However, if the first hurdle is passed and the Stray manages to find a home, then the greatest danger is passed. Unlike Summoned Ones, the Strays are not subjected to an enslavement ritual and thus have greater freedom, however likewise there is no guarantee of obtaining a Unique Skill in turn, and in general, there is a chance for these kinds of Otherworlders to be comparatively weak like in the case of Yoshida, however that is not guaranteed either. For example, Hinata Sakaguchi obtained 2 Unique Skills despite being a Stray.