disappearance (Noun) — The act of leaving secretly or without explanation.
disappearance (Noun) — The event of passing out of sight.
disappearance (Noun) — Gradually ceasing to be visible.
disappearance (Noun) — Ceasing to exist. ex. "he regretted the disappearance of Greek from school curricula" ex. "what was responsible for the disappearance of the rainforest?" ex. "the disappearance of resistance at very low temperatures"