The word biàntài literally combines biàn (to change) and tài (form, state), so its core meaning is 'change of form' — a precise scientific term in biology for metamorphosis, as seen in insects (e.g., caterpillar → butterfly) or amphibians (e.g., tadpole → frog). In this neutral, technical sense, it’s commonly used in textbooks and nature documentaries.

In colloquial speech, however, biàntài has taken on a strongly negative, informal connotation meaning 'abnormal', 'perverted', or 'disturbing', often describing behavior, preferences, or situations that deviate sharply from social norms. This usage is emotionally charged and should be avoided in formal or polite contexts. The shift reflects how scientific terminology can acquire slang meanings through frequent, exaggerated use — much like English 'sick' or 'wicked'.