Generic Units is a fan term for units without a portrait or real name. Generic units typically have relatively low stats compared to named characters and other attributes. They also usually lack more complicated features like Affinities and Biorhythm. In the earlier titles and their remakes, generic units were also distinguished by their lack of Luck and lower weapon levels.
Generic units tend to be named after the faction they belong to, though in some games such as The Sacred Stones they are given more general names such as "Soldier" or "Monster". In many Fire Emblem games, generic unit portraits are impersonal and focus on showcasing how their class fights. In these games, generic units will often use specific portraits in the rare times they speak. These speaking portraits are often based on the Soldier class regardless of the actual class of the speaker.
A few notable generic units have appeared in the series, however.
Generic units usually consist of males unless it is a female-dominated class like Pegasus Knight or Troubadour. The main exceptions to these are the Fire Emblem: Genealogy of the Holy War, Thracia 776, and Three Houses.