Acausality is defined as the ability to exist outside of causality, or the natural flow of manipulation defined by cause and effect.
An entity that is acausal has no true beginning or end, and is thus essentially immune to offensive causality manipulation and time travel. Even if you go back in time and kill an acausal being in the past or prevent him/her/it from being born/created, it will still exist in the present and other timelines. Often, even if an acausal being is killed in the present, it can still survive by appearing from another timeline. Thus, acausal beings are very difficult to permanently destroy, often requiring the use of high-level reality warping.
Note: This page is based on Causality Immunity by the Superpower wiki.