The main character is the last living example of homo sapiens on a world devoid of other sentience, either terrestrial, extra-terrestrial, or artificial.
At some point in the narrative, the main character is presented to the reader or thinks him/herself to be the last living homo sapiens. The narrative contains the primal scenes of the Last Man genre. The character eventually comes in contact with one or more sentient beings. The stories vary in length of time before contact, the nature of that contact and the results of that contact. A critical difference is also to be found in the number of people left alive that are eventually discovered.
Stories that use gendercide to drive the plot. While not all gendercide in fiction results in a Last Man (or woman) story, some are pitched directly as that by using the phrase.
Stories that whittle a group of characters down to an Adam and Eve Shaggy God plot, technically resulting in a “Last Man.” This is to be differentiated from 2nd order stories where a classic Last Man meets a surviving Last Woman.
Stories that are a pun on the phrase “Last Man on Earth.” Also stories that refer to the Biblical notions of the "Last Man," i.e. the generation that will witness the return of Christ.