Which instrument helps to indicate a pilot's altitude during flight?

The altimeter is the instrument specifically designed to indicate a pilot's altitude during flight. It works by measuring the atmospheric pressure around the aircraft and translating that pressure into an altitude reading based on established standards of pressure relative to sea level. As altitude increases, the surrounding air pressure decreases, and the altimeter reflects this change. This allows pilots to maintain appropriate altitude levels for safety and navigation.

In contrast, the vertical velocity indicator measures the rate of climb or descent rather than providing an absolute altitude. The magnetic compass helps with navigation by showing the direction relative to magnetic north, but it does not indicate altitude. The turn-and-slip indicator is used to assess the coordination of turns and help maintain altitude during turning maneuvers, but it does not provide any altitude information. Hence, in the context of altitude indication, the altimeter is the essential instrument for pilots.