The description of the word you requested from the astronomical dictionary is given below.
'pla‧net, plural planets; planètès astèr = [Greek] wandering star
A planet is a large spherical object with a diameter between about 1000 and 300,000 km. A planet is massive enough that its own gravity keeps it round (in contrast to asteroids, comets, and small moons), but too small to generate energy in its core through nuclear fusion (in contrast to stars). A planet orbits directly around a star (in contrast to moons) and has no similar other objects near its orbit (in contrast to dwarf planets).