Octree is a method of dividing three dimensional space.
Space is repeatedly divided into eight divisions. Each divided space is called an octant.
For example, when a cuboid is divided using octree, the cuboid is repeatedly divided at the center of each side (as shown below).
scFLOW and SC/Tetra generates unstructured mesh elements by creating groups of cuboids with octree and by adjusting mesh scale and coarseness.