What does sinusoidal mean?
"Sinusoidal" means a movement that goes in one direction, then gradually slows down, stops and changes into an opposite direction, then again slows down, stops and changes into an original direction. This cycle repeats itself.
Presented on the time line (X-axis) with directions of movement along the Y-axis, this behavior resembles the graph of a trigonometric function
The movement described above is not necessarily a mechanical movement. It can be a change in the temperature, pressure, even the person's mood going from excitement to depression and back.