grmljavina

1. thunder; thunderstorm
   Dete se probudilo od grmljavine. — The child woke up because of the thunder.
   Za sutra se najavljuju kiša i grmljavina. — Rain and thunderstorms are forecast for tomorrow.
2. (of loud noise) roar; rumble
   Čula se grmljavina topova iz daljine. — The roar of cannons could be heard in the distance.

"Grom" is a single thunderclap or lightning strike, "grmljavina" is the ongoing rumbling.