Is it considered an accusation of unchastity if I describe a woman as an "adulteress" to myself or with my tongue, especially if no one hears me? And is repentance required for that?

If what one feels inside is merely a passing thought and is resisted, it does no harm; rather, the one who resists it is rewarded. Accusing someone of adultery in private is not considered slander (qadhf) that necessitates the legal punishment (hadd), but rather it is a minor sin, because the reason for the ruling on slander is the infliction of disgrace, which is absent in private. What is incumbent upon the one who falls into this is to repent to Allah Almighty. One must resist and ignore obsessive thoughts (waswasah), and if these thoughts become overwhelming and overpower a person, then they are not held accountable for what issues from them because of it.