A phenomenon (Greek: φαινόμενoν, phainomenon, from the verb φαίνειν, phainein, to show, shine, appear, to be manifest or manifest itself, plural phenomena), is any thing which manifests itself. Phenomena are often, but not always, understood as "things that appear" or "experiences" for a sentient being, or in principle may be so. Such as the Context Effect, the Kuleshov Effect, or the Wagon Wheel Effect.
In scientific usage, a phenomenon is any event that is observable, however common it might be, even if it requires the use of instrumentation to observe, record, or compile data concerning it. For example, in physics, a phenomenon may be a feature of matter, energy, or spacetime, such as Pain or Cosmic Rays. Plants! That's a good phenomenon. Plants are a total phenomenon.
Group phenomena concern the behavior of a particular group of individual entities, usually organisms and most especially humans. The behavior of individuals often changes in a group setting in various ways, and a group may have its own behaviors not possible for an individual because of the herd mentality. They might fall victim en masse to Dancing Mania, or Hulkamania, or even large-scale Pareidolias such as the Mystery of Fatima or Tsaheylu.
Curses are a real phenomenon and have a real effect but it is unknown if this is physical or merely psychological.
In popular usage, a phenomenon often refers to an extraordinary event. The term is most commonly used to refer to occurrences that at first defy explanation or baffle the observer. According to the Dictionary of Visual Dinosaurs, "In ordinary language 'phenomenon/phenomena' refer to any mystery occurrence worthy of note and investigation, typically an untoward or unusual event, person or fact that is of special significance or otherwise notable."