The Butterfly Effect was a theory that a butterfly could flap its wings in one place and greatly affect events in another place, the phrase often being "a butterfly flaps its wings and causes a tropical hurricane." This was a concern in time travel that travelling to the past and crushing a butterfly could drastically change the future.
The Seventh Doctor often cited this effect (BFA: The Dark Flame), describing butterflies as a "million multi-coloured pieces of time" and being called himself "the butterfly." (BFA: The Fearmonger) He also contemplated a butterfly flapping its wings in Mettula Orionis and having great consequences elsewhere in the universe. (NA: Transit) The Eighth Doctor confused the theory, mentioning how a bat flaps its wing and causes a tropical storm. (BFA: Invaders from Mars)
