Plain English Meaning
Things don't start moving, stop moving, or change direction on their own. Some force has to make it happen.
Full Definition
Newton's First Law of Motion, which states that a body at rest tends to remain at rest, and a body in motion tends to remain moving in a straight line at the same speed, unless acted upon by an outside force.
Context Anchor
Seen in aerodynamics and instrument flying discussions when explaining why an aircraft keeps doing what it is doing until a force changes its motion.
Derivation
Inertia comes from the Latin iners, meaning 'inactive' or 'sluggish.' It captures the idea that matter, left alone, has no tendency to change what it's already doing.
Why Pilots Care
Understanding this law explains why an aircraft maintains straight-and-level flight or a turn without constant and why must overcome existing motion to change or direction.