Ability was the inherent power or capacity of someone or something, which made it possible for them to cause a desired outcome. There was a range in the magnitude of an ability, from limited, or inferior, to extraordinary, or superior. The opposite of ability was inability. (TOS: "Where No Man Has Gone Before", "Space Seed"; TNG: "Datalore", "Darmok", "Birthright, Part I"; VOY: "Time and Again")
An individual was born with inherited natural abilities from their parents and acquired new abilities by learning skills, which took years of experience to develop, or through genetic enhancing. Adding a new ability or enhancing an existing ability could result in the alteration of their genetic structure. (TOS: "Where No Man Has Gone Before"; TNG: "The Loss"; DS9: "Equilibrium", "Doctor Bashir, I Presume"; VOY: "Prototype"; ENT: "Cold Station 12")
In some situations, the right to have an ability was a privilege subject to approval by the authorities. For an example, on board the USS Voyager, The Doctor successfully asked for the ability of turning his program on and off from Captain Kathryn Janeway. (VOY: "Revulsion")
An organic individual was born with the same abilities as everyone else in their species or any other organic species. These were the ability to assimilate (consume food and derive energy from it), respirate, reproduce, grow and develop, move, secrete and excrete, adapt to their surroundings, and regenerate. There were inherited natural abilities which were species-centric, like the telepathic ability in Vulcans or the empathic ability in Betazoids. (TOS: "A Taste of Armageddon"; TNG: "Home Soil", "The Loss", "The Quality of Life"; VOY: "Dark Frontier")
An artificial individual was given their abilities by their creator. (TNG: "Brothers")
An individual identified by others as a genius had abilities which went beyond the understanding and abilities of people who were not geniuses. (TNG: "Where No One Has Gone Before")
In the 24th century, the holodecks opened new avenues for adding and developing abilities, as The Doctor demonstrated aboard the USS Voyager in the 2370s when he programmed exercises for social graces. In his opinion, this benefited him greatly for he was rewarded with the "ability to put others at ease" and "make them feel comfortable around" him. (VOY: "Prey")
An android designed for receiving the consciousness of an individual through synaptic fusion would have the same abilities as the original. (TOS: "What Are Little Girls Made Of?"; TNG: "The Schizoid Man", "Inheritance"; PIC: "Et in Arcadia Ego, Part 2")
Individuals felt pride in their abilities. (TNG: "Hero Worship"; DS9: "Heart of Stone")
In the 2250s, in the alternate reality, Spock described Nyota Uhura as having "an unparalleled ability to identify sonic anomalies in subspace transmission tests". (Star Trek Into Darkness)
In 2256, Ambassador Sarek of Vulcan was described by Admiral Terral as having a "unique ability to forge relationships with races that do not follow logic-based ideology." (DIS: "Lethe")
In 2365, First Officer Flaherty of the USS Aries was described by Captain Jean-Luc Picard as having the "unique ability of instantaneously interpreting and extrapolating any verbal communication that he hears." (TNG: "The Icarus Factor")
That same year, Commander William Riker informed Captain Jean-Luc Picard that one of his strengths was the "ability to evaluate the dynamics of a situation, and take a definitive preemptive step, take charge." (TNG: "Time Squared")
In 2367, Data, in a letter to Bruce Maddox, spoke of his admiration for the fictional detective Sherlock Holmes and "his ability to solve mysteries by careful examination of the available evidence." (TNG: "Data's Day")
That same year, Miles O'Brien was described by Captain Benjamin Maxwell as having the ability "to size up a situation instantly, then come up with options to fit all contingencies." (TNG: "The Wounded")
In 2368, in a conversation with Captain Jean-Luc Picard, Spock described himself as having the ability for seeing beyond pure logic. Spock's father Sarek considered it weak, though his son found that it was a source of extraordinary strength. (TNG: "Unification II")
In 2369, the clone of Kahless assumed his role in the Klingon Empire as a figurehead with the "ability to rally the people, to lead by example, to guide them in spiritual matters." (TNG: "Rightful Heir")
In 2373, Garak described Dukat as "just another swaggering, self-important gul with too much vanity and not enough ability." (DS9: "Things Past")
Short-term loss of an ability could occur when an individual suffered from an injury or illness or had it temporary disabled by a medical practitioner. Long-term loss of an ability could occur through disuse over a long period of time. (TOS: "Where No Man Has Gone Before"; TNG: "The Loss"; VOY: "Caretaker", "Meld")
One of the ways for preventing the loss of ability was the constant practice of it by an individual. Another way was an individual availing themselves of any opportunity to test their abilities. In Starfleet, the no-win situation tested the abilities of officers. (TNG: "Haven", "Thine Own Self"; DS9: "The Magnificent Ferengi")
When an individual was relearning a long disused ability, they might need an experienced guide for helping them along the journey of rediscovery. In the initial stages, when the individual was undisciplined with their abilities, the guide would help the individual with focusing so they could achieve something. Later, after much practice and learning control, they might be persuaded by their guide into using a series of guided meditations which would help the individual in exploring the depths of their ability. (VOY: "Cathexis", "Cold Fire", "The Gift")
Expansion of an ability came when its limits were tested by a problem. The abilities of Captain Jean-Luc Picard were tested to their limits when he solved a problem created by Q and, for a brief moment, his mind and his horizons expanded. (TNG: "All Good Things..."; VOY: "Think Tank")
One of the dangers associated with being given abilities of extraordinary magnitude was that the individual might feel that they were different from and better than others in their species. As for these others, they might not comprehend or understand the abilities being demonstrated. They would see the individual as mutating into a new form of life or as a monster. This individual might come to view these "lesser individuals" as toys for their amusement or as pets. (TOS: "Where No Man Has Gone Before"; TNG: "Hide And Q"; VOY: "Cold Fire"; ENT: "Observer Effect")
In Starfleet, the progression through ranks was designed to compensate for the limitations of Human abilities. It was expected of each member in the organization that they would participate and advance according to their abilities. These members were united as a team, which remained strong so long they had the ability of pooling their talents. Within each crew, a member received a task best suited to their abilities. During briefings, the superior officer would get answers based on each officers' abilities and experience. Some superior officers consider it important that their key officers know each other's abilities. In this friendly environment, the individual might be told by another that they had complete or full faith and confidence in their ability to get something done. On the other hand, an individual might be told by another that they did not believe in or had doubts about their ability to get something done. (TOS: "The Naked Time"; TNG: "Encounter at Farpoint", "Datalore", "The Measure Of A Man", "Devil's Due", "Redemption II"; DS9: "The Siege", "In Purgatory's Shadow"; VOY: "Year of Hell", "Tinker Tenor Doctor Spy", "The Omega Directive")