(meteorology) the vertical movement of heat or other properties by massive motion within the atmosphere WordNet 3.0 & Open English WordNet

The vertical movement of heat and moisture, especially by updrafts and downdrafts in an unstable air mass. The terms convection and thunderstorm are often used interchangeably, although thunderstorms are only one form of convection. Towering cumulus clouds are visible forms of convection. Wiktionary

Vertical movement of heat and moisture. Wiktionary (translation)