Planet X was a hypothesis created by Percival Lowell to help explain anomalies in the orbits of the gas giants. He speculated that "the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the orbital irregularities." In 1930 the planet Pluto was discovered. Many thought this discovery proved Lowell's hypothesis. It was not until the early 1990s that Voyager 2 identified "that the irregularities observed in Uranus's orbit were due to a slight overestimation of Neptune's mass."
Now in popular culture, and even among some astronomers, Planet X has become a "stand-in" term for any undiscovered planet in the outer
Planet X is real to a point it is the planet that has not been recourded but is like in math for example in problems thier are X's or variables and we do not know what they are till we figure out the problem just like we need to figure out space to find more planets and once we find one it is Planet X.
Planet X is the name that was given to yet to be discovered unknown planet. The possibility of existence of such a planet was based o some apparent discrepencies observed in the orbits of gas giants Uranus and Neptune. Perceival Lowell proposed his Planet X hypothesis in early twentieth century based on this. For many years after that the existence of Planet X was considered a by reputed scientists to be a very likely reality. Discovery of Pluto in 1930, further increased the belief of scientists in reality of Planet X. Many attempts were made to locate this planet and to determine its details. However no conclusive evidence could be found. Finally in 1990 a study of measurements made by the Voyager 2 established that irregularities observed in Uranus's orbit were due to overestimation of Neptune's mass. With this serious speculation among scientific community about existence of Planet X came to and end.
Now scientist agree that Planet X, as envisaged originally, does not exist. However the term Planet X is used by many scientist for any other undiscovered planet in the outer Solar System.