A stratified fluid has some layers stacked vertically due to changing temperature (as is the case in atmosphere) and changing density or both (as is the case in ocean). Typically the temperature or density decreases as we go up, in which case we say the fluid is "stably stratified". However since these system experience a lot of external energy input (think radiation, waves or circulation), the fluid is pushed around and is essentially in a turbulent state. This idealized system can be studied theoretically / computationally (in a periodic box) and experimentally (in a salty water tank) as an analogue for the atmosphere and ocean.