The correct answer is - isobar.
The isobars are closed lines that are connecting spots with the same air pressure, thus giving us information about the air pressure in certain places. The air pressure can be standard, low, or high. The standard air pressure is 1013.25 mbar, the low air pressure is bellow the value of the standard one, and the high air pressure is above the standard one.
The low air pressure means that there are warm air masses in the area, or more water vapor in the atmosphere, while the high air pressure means that there are cold air masses present in the area. The reason why the warm air masses have low air pressure is that they are less dense, thus have less mass, and make less pressure, while the cold air masses are denser, have more mass, and make more pressure.
The unit most commonly used for measuring the air pressure is milibar (mbar)
