What is the movement of particles from a high concentration to a low concentration called?

1. A drop of food colouring slowly spreads out through a glass of water. 2. Diffusion is the movement of particles from a high concentration to a low concentration. 3. This spreading only happens because matter is made of particles that keep moving. _Source: Siyavula Physical Sciences Grade 10 (Everything Science, CC BY 4.0), Chapter 3: States of matter and the kinetic molecular theory_