Distance
Distance is the total length of the path an object travels, a scalar quantity measured in meters with no direction.
Definition
Distance is how far something travels along its path, counting every step, turn, and loop, measured in units like meters or miles. It is a scalar quantity, meaning it has magnitude only and no direction, and its SI unit is the meter (m); distance divided by time gives average speed. In calculus terms, distance is the arc length of an object's trajectory, $s = \int |\vec{v}|\, dt$, the integral of speed over time. Because speed is never negative, distance is a monotonically non-decreasing function of time. Distance equals the magnitude of displacement only when the motion is along a straight line in one direction.
Example
If you walk $40\ \text{m}$ to the water fountain and $40\ \text{m}$ back to your seat, you traveled a distance of $80\ \text{m}$. A hiker who walks $3\ \text{km}$ north and then $4\ \text{km}$ east has covered a distance of $3 + 4 = 7\ \text{km}$, even though the hiker is only $5\ \text{km}$ from the starting point in a straight line. A pendulum bob that swings through an arc of $0.20\ \text{m}$ from one side to the other travels $20 \times 0.20 = 4.0\ \text{m}$ of distance after ten full swings (twenty one-way trips), yet its displacement from the starting side is zero.
Key Insight
Distance never goes down. Even if you walk in a circle and return to where you started, the distance you traveled keeps adding up, which is exactly what your car's odometer does: it does not care whether you drove in circles or in a straight line, it just totals the road under the wheels. This path-dependence has a physical consequence. Work done by friction depends on distance, not displacement, because friction opposes the motion at every point along the path, and that is why friction is a non-conservative force.