Turbulence is small-scale and short-term irregular motion of the fluid (the air), in the course of which fluid particles move chaotically. When the fluid moves at a subcritical speed, the flow is regular (ie. laminar), but when that speed passes a critical value, the flow becomes chaotic or turbulent.
Turbulence is a major aviation hazard and, depending on its force and dimensions, it may range from mere jostling of the aircraft to the turbulences that may cause a crash of the aircraft (the smaller ones in the first place). Turbulences appear in the clouds and their vicinity, especially in thunderstorm clouds.
Quite unpleasant type of turbulence is clear air turbulence (CAT), since the pilot cannot detect it before flying into it. Although CAT is not considered to be extremely dangerous, it has an impact on the comfort of the flight. There were some cases when passengers and crew members were injured or even lost their lives due to strong turbulences encountered.