In 2006, the International Astronomical Union (IAU) declared that Pluto was not a planet, but rather a dwarf planet. This was largely because Pluto is relatively small (smaller than our moon) and it belongs to the Kuiper Belt—a body of comets/large objects that orbit the sun on the outer edges of the solar system. This presented a problem for the IAU; but to see why, we must first see how the IAU defines a regular, full-size planet.
The three main criteria for a full-size planet are that it orbits the sun, is (almost) spherical in shape, and has “cleared its neighborhood.” By “clearing its neighborhood,” the IAU means that a planet must not orbit in sync with nearby, similar objects. Rather, all such objects must be gravitationally attracted to the planet. This is where Pluto fails. Because there are other, relatively similarly-sized objects in the Kuiper Belt that are not affected by Pluto’s gravity, it has not “cleared its neighborhood.”
While the IAU maintains this important distinction between regular planets and dwarf planets, planetary scientist Alan Stern offers an interesting perspective—that the title is merely semantics. He states how, according to planetary scientists, all dwarf planets are planets by nature. A dwarf planet is simply a small planet. Therefore, it is irrelevant and misguided to characterize Pluto as “not a planet.”
Here is a diagram from ESA Hubble comparing the sizes of the largest Kuiper Belt objects, emphasizing how Pluto has not “cleared its neighborhood” of comparably-sized objects.