What is Geostrophic wind?

Hint: In a rotating system, geostrophic motion is fluid movement in a direction parallel to lines of equal pressure (isobars).

Complete answer:
The balance of the horizontal pressure gradient pressure and the force due to the earth's rotation to the left in the northern hemisphere and to the right in the southern hemisphere determines the direction and speed of the wind. The horizontal wind velocity at which the Coriolis force balances the horizontal pressure gradient force perfectly. The geostrophic wind is therefore directed to the left in the northern hemisphere and to the right in the southern hemisphere along the contour lines on a constant-pressure surface (along the isobars on a geopotential surface) with low altitudes (low pressure).

Example-An artillery round launched from a vast distance is a frequent example. If the coriolis force is not taken into consideration, it will land to the right (in the northern hemisphere) of the predicted route. The shell seems to bend to the right to an observer on the earth's surface, despite being on a ballistic arc.

Cause-The Coriolis force deflects the air mass to the right as it begins to move. The deflection rises until the pressure gradient force equalises the Coriolis force. The wind will be blowing parallel to the isobars at this moment. The wind is referred to as "geostrophic wind" when this happens.

Note: Wind gets its initial speed and direction from pressure gradient force, which is caused by variations in air pressure over time (PGF). This is why wind speeds tend to rise when isobars are packed closer together (greater pressure gradient).