A phenomenon (plural phenomena) was a physical and measurable something. They were found on planets and in space. A phenomenon was gauged by its size and period. For example, a local phenomenon had a small size and a short period. Some anomalies were phenomena. When in doubt on whether a phenomenon occurred in an illusion or in real time, an individual could check to see if machines like computers were affected. If they were affected, then the event happened in real time. (TOS: "The Alternative Factor"; TNG: "We'll Always Have Paris", "Remember Me", "Clues", "A Matter Of Time"; VOY: "Twisted")
Most phenomena conformed to the laws of physics. For example, the three laws of motion as set out by Isaac Newton in the 17th century. When the movement pattern of a natural phenomenon did not conform to these laws, it might suggest what was being studied could be a living being. Natural phenomena could not travel faster than the speed of light. (TOS: "The Lights of Zetar"; TNG: "Tin Man", "The Loss"; VOY: "Twisted")
The interconnected nature of phenomena and anomalies was one of the subjects studied in spatial causality. As an example, it was not possible to create an anomaly without causing a phenomenon which would, in turn, affect a civilization. (VOY: "Q2")
In his assessment of Talur of Barkon IV's scientific reasoning of the universe, Data said, "I believe you are reasoning by analogy, classifying objects and phenomena according to superficial observation rather than empirical evidence. Wood, for example, does not contain fire simply because it is combustible, nor does it contain rock simply because it is heavy. Wood, like any complex organic form, is composed of thousands of different chemical compounds, none of which is fire." (TNG: "Thine Own Self")
Some phenomena, like subspace sinkholes and quantum singularities, were mono-directional. (VOY: "Gravity")
Some phenomena were more common than others. Dichromic nebulae, gravimetric distortions, and class 4 pulsars were relatively common. Others were rarer, like wormholes. The odds of finding a wormhole were infinitesimal. (TNG: "Bloodlines"; VOY: "Bliss")
A major class of phenomena was natural phenomena. Subcategories of natural phenomena included astronomical phenomena (such as stellar phenomena), subspace phenomena, and cultural phenomena. Practical jokes were an example of a cultural phenomenon. Cultural phenomena were studied by anthropologists. (TOS: "The Alternative Factor"; TNG: "Tin Man", "The Game"; VOY: "Emanations", "The Q and the Grey", "Counterpoint", "Tsunkatse")
Reflection was a quality of some natural phenomena. (TOS: "Elaan of Troyius"; TNG: "Hero Worship")
In an act of deception to prevent the USS Pegasus from being discovered by the Romulans, the asteroid which contained the wrecked ship was saturated with ionizing radiation by the USS Enterprise-D. This fooled the Romulans into thinking there was a natural phenomenon near or on the asteroid. (TNG: "The Pegasus")
A natural phenomenon like electromagnetic interference in a planet's magnetosphere hindered the ability of sensors to scan the surface. (TNG: "Descent")
A natural phenomenon with biological properties was a lifeform. (TNG: "Galaxy's Child")
In 2368, after a Type C asteroid had struck an unpopulated continent on the planet Penthara IV, the resulting dust cloud could have created a phenomenon not unlike the nuclear winters of 21st century Earth. (TNG: "A Matter Of Time")
After a natural phenomenon, such as bioforming, entered an individual's body, it could be undone by a medical practitioner if they had enough information. (VOY: "Demon")
As of 2373, there were 5,021 medical records on space plasma phenomena. (DS9: "Things Past")
Another type of natural phenomenon was a psionic phenomenon. This phenomenon could be telepathic in nature. When this phenomenon occurred, it stimulated mental activity in the brains of humanoids. (DS9: "The Muse")
The Cardassian Garak was interested by a sociological phenomenon among Humans. "For generations now, humankind has had more than enough food and yet you go about your eating as if you were afraid someone was going to come along and snatch away your plate." (DS9: "Improbable Cause")
For many centuries, spacecraft and outposts had either been damaged or lost to natural phenomena. (TAS: "The Time Trap"; TNG: "The Neutral Zone", "Lessons"; VOY: "Alice", "One Small Step"; ENT: "The Catwalk")
Another major class of phenomenon was the mechanically created phenomenon. This phenomenon bore the evidence of the intelligence who created it. (TOS: "The Alternative Factor"; VOY: "Twisted"; ENT: "Doctor's Orders")
Finally, there was the class of temporal phenomena. (VOY: "Coda")
Federation starships had files on all known phenomena in their databanks and were constantly adding information on new phenomena to those databanks. When a starship charted a new phenomenon or experienced it only once, it was impossible for their crew to estimate the risks. Information about a phenomenon was learned from sensor readings and computer analyses. The personnel on board these ships were trained to accept that often a scientific analysis of a seemingly impossible phenomenon would prove it to be possible. (TOS: "Turnabout Intruder"; TNG: "Imaginary Friend"; VOY: "Twisted", "The Fight")
In 2370, the Federation tested the interface probe. The probe permitted Geordi La Forge to sense phenomena which were not perceptible to any other kind of sensor. (TNG: "Interface")
In 2376, interstellar phenomena forecasts were broadcast by Deep Space 9. In one of the forecasts, it was predicted that a Class B itinerant pulsar would pass within four billion kilometers of the MIDAS array. (DS9: "Pathfinder")