What do scientist mean when they use the term inertia?

The term inertia means a property of matter by which it continues in its existing state of rest or uniform motion in a straight line, and less that state is changed by an external force

The word inertia is the resistance of any physical object to any change in it's state of motion. Things like the objects direction, speed, or it's state of rest are all included.

Inertia can also be defined as the tendency of objects to keep moving in a straight line at it's constant velocity.