‘Annihilation’ is a foundational concept in the Holipsych.com Theory. We use the term in a very different meaning from ‘human extinction, mutual destruction’ (Wikipedia).
In this theory, ‘Annihilation’ is a personal, inner (and usually subliminal, not fully conscious) perception or experience some people have. The overlap in meaning with ‘human extinction, mutual destruction’ is that the person feels destroyed, reduced to zero, due to not being seen, heard and taken into account. It means feeling as if one is alive but doesn’t exist to others. It means experiencing not having a voice or a face as a ‘me’, not being acknowledged as an independent, potentially autonomous human being.
Be sure to read now, if you haven’t already, the companion term ‘Fear of Annihilation’ here.