Nudge open the valve to your garden hose and you’ll see clear water stream out of the hose slowly and smoothly. The stream looks orderly because on the microscopic level, it is. Inside the stream, layers of water move over one another with little to no mixing. Engineers who’ve learned fluid dynamics know that this process, called laminar flow, is more common in viscous fluids and when fluids are flowing slowly, and it is possible to analyze mathematically. It’s also relatively rare in nature.

Turbulent flow is much more common. If you open the valve feeding the hose enough, the water flowing out will start to oscillate and break up and move randomly and chaotically. Inside that stream, some layers of water are spinning out into whirlpools and eddies, just like you’d see in a whitewater stream. Dyes in the water can help visualize this phenomenon, called turbulence, but quantitatively analyzing turbulent flow remains a challenging, difficult and as yet unsolved problem in fluid mechanics, says Vrishank Raghav, an assistant professor of aerospace engineering at Auburn University.

In cardiology, turbulence can literally mean the same thing. In most blood vessels, blood flows via laminar flow, but where big arteries like the aorta branch, flow can be turbulent. Flow can also be turbulent where arteries are constricted from atherosclerosis. A doctor can hear signs of turbulent flow through a stethoscope, which can help diagnose the disease.

A cardiologist who discusses turbulence might also be talking about something entirely different. She might be referring to variations in heart rate, where the heart speeds up for a few beats, then slows back to baseline. Cardiologists call that phenomenon heart rate turbulence. Cardiologists have learned that patients with lower turbulence have a higher risk of dying from heart disease.

Most physicians who mention turbulence at all do not have such a specific meaning in mind. They just want to diagnose disease, treat patients, and save lives. “To a doctor, turbulence means that blood flow in the heart is not orderly, and this has a connection to a clinical problem,” Raghav says.