The description of the word you requested from the astronomical dictionary is given below.
convectio = [Latin] bring together, from con- = with, and vectio = carry
Convection is a form of energy transport in which the material with the energy in it moves. The hotter material moves toward the cooler area and the cooler material toward the hotter area. On Earth, you can see convection in a pan of boiling water (where hot water moves up and cooler water moves down), and also in a thunderstorm (where warmer, moist air moves up and forms clouds). You can also find convection in the convection zone of the Sun, and in granulation.