thunder:The loud noise accompanying a flash of lightning (apparently following it, being heard after it at an interval depending on distance), due to the sudden violent disturbance of the air by the electric discharge; varying from a sharp report or crash to a prolonged roll or reverberation. Also, the unseen cause of the phenomenon, the meteorological condition or action (scientifically, the electric storm and discharge) from which the loud noise proceeds.——Oxford English Dictionary

thunder:the loud noise that you hear during a storm, usually after a flash of lightning.——Longman Dictionary of Contemporary English  @en