The term "Planet X" has a variety of meanings and uses. After the discovery of Neptune, Percival Lowell "proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the giant planets, particularly Uranus and Neptune, speculating that the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the irregularities."
Today, the astronomical community widely agrees that Planet X, as originally envisioned, does not exist, but the concept of Planet X has been revived by a number of astronomers to explain other anomalies observed in the outer Solar System. In popular culture, and even among some astronomers, Planet X became a stand-in term for any undiscovered planet in the outer Solar System, regardless of its relationship to Lowell's hypothesis.This week, good evidence was reported for the existence of such a previously-undiscovered planet. I've seen it referred to as Planet X or as Planet IX.
The embedded video gives a broad overview. This article in Science provides more details. Even more amazing than the existence of the planet is its extraordinary orbit:
In the schematic, the entire known solar system is represented by the small blue circle in the center (magnified in an offset in the upper part of the image).
The orbit of the inferred planet is similarly tilted, as well as stretched to distances that will explode previous conceptions of the solar system. Its closest approach to the sun is seven times farther than Neptune, or 200 astronomical units (AUs). (An AU is the distance between Earth and the sun, about 150 million kilometers.) And Planet X could roam as far as 600 to 1200 AU, well beyond the Kuiper belt, the region of small icy worlds that begins at Neptune’s edge about 30 AU.Mind-boggingly awesome.