The description of the word you requested from the astronomical dictionary is given below.
dwarf planet, plural dwarf planets
A dwarf planet is a large spherical object with a diameter of around or just above 1000 km. A dwarf planet is massive enough that its own gravity keeps it round (in contrast to asteroids, comets, and small moons), but too small to clear the neighborhood of its orbit from similar objects (in contrast to planets). A dwarf planet orbits directly around a star (in contrast to moons).