Rapid means something which occurs quickly (fast), As in, "he flipped through the dictionary rapidly" Rapid also refers to a section of a river where the river bed has a relatively steep gradient causing an increase in water flow and turbulence.
This defines a rapid population increase.
A rapid reaction between oxygen and a fuel is called combustion.
A rapid change in temperature with depth in the ocean is called thermocline. A rapid change in density with depth in the ocean is called the pynocline.Thermocline
Rapid Eye Movement
The rapid growth of a population of algae is called an algal bloom. This bloom is often triggered by an abundance of nutrients in the water, such as nitrogen and phosphorus, leading to exponential growth of algae species. Algal blooms can have negative impacts on aquatic ecosystems, including depleted oxygen levels and harm to wildlife.
The 1920s was a decade of rapid industrial growth such growth is called the Roaring Twenties.
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Sudden, rapid movements in glaciers are called glacier surges. Narrow, fast moving sections of an ice sheet are called ice streams.