Invisibility

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Marcie watches as her hand fades out of sight

Invisibility was the power or condition of not being seen by other beings. 

Certain beings could be rendered unseen while others were just merely taken out of the perception of specific beings.

Cases

Scientific

• The trio invented an invisibility ray, which they used on themselves (Also unintentionally turning Buffy invisible). However, the after effects of being turned invisible included the destruction of the subject's molecular structure, with the result that the invisible subject would eventually collapse if not restored to normal.

Demonic

• Vocah could turn invisible at will.
• Der Kindestod was naturally invisible to all but the sick.
• The Thesulac Demons and Pockla Demons could turn invisible.

Magical

• The Hellmouth caused Marcie Ross' feelings of neglect to manifest. This was based on the idea of reality being shaped by perception; because many students perceived Marcie as invisible, she literally became invisible.
• The effects of Gachnar's spell caused Xander Harris to be invisible to his friends.
• Tara Maclay accidentally made a group of Lei-Ach Demons invisible to the Scooby Gang when casting a spell that prevented them from seeing demons (as her goal had been to conceal her perceived demonic heritage from the gang).
• Rack had his residence cloaked; it was only detectable by a vampire, demon, or witch through a rise in ambient temperature
• Willow Rosenberg subconsciously cast a spell to make Buffy, Xander and Dawn unable to see her and vice versa due to her guilt over her past actions.
• Rupert Giles noted that there were various forms of invisibility in the world, but did not elaborate on other potential causes; Xander Harris mentioned the invisibility cloaks of Greek gods, but it is uncertain how accurate this statement was.

Other 

• According to Xander Harris, Greek myths talk about cloaks of invisibility, but they're usually just for the gods.
• The ghost Maude Pearson could make herself invisible to some, while her son could not take physical form.
• The First could choose who to appear to and remain unseen and unheard by others.

Invisible Individuals

• Marcie Ross
• Invisible students
• Der Kindestod
• Vocah
• Xander Harris (temporarily, twice)
• "Baby" and his friends (temporarily)
• Spike (temporarily)
• Buffy Summers (temporarily, twice)
• Warren Mears (temporarily)
• Jonathan Levinson (temporarily)
• Andrew Wells (temporarily)
• The First Evil
• Willow Rosenberg (temporarily)
• Dawn Summers (temporarily)
• Shy Girl (temporarily)

Appearances

• "Out of Mind, Out of Sight"
• "Killed by Death"
• "Amends"
• "Fear, Itself"
• "To Shanshu in L.A."
• "Are You Now or Have You Ever Been"
• "Family"
• "Dead End"
• "Wrecked"
• "Gone"
• "Same Time, Same Place"
• "Storyteller"