* Rectilinear motion: This refers to motion along a straight line.

* Simple harmonic motion: This is a specific type of oscillatory motion where the restoring force is directly proportional to the displacement from the equilibrium position, and the force always acts towards that equilibrium position.

While SHM can be visualized as oscillating back and forth along a line, the motion itself is still linear. This means the object in SHM is always moving in a straight line, even if it changes direction periodically.

Examples:

* A mass on a spring: The mass moves back and forth along a straight line.

* A pendulum swinging with small angles: While the pendulum traces a circular arc, its motion along the arc can be approximated as linear for small angles.

Key takeaway: Simple harmonic motion is a type of linear motion because the object's movement is always along a straight line, even though it oscillates.