Viscose, which is also commonly known as rayon when it is made into a fabric, is a type of semi-synthetic fabric. The name of this substance comes from the process that's used to make it; at one stage, rayon is a viscous, honey-like liquid that later settles into a solid form.
The primary ingredient of rayon is wood pulp, but this organic ingredient goes through a lengthy production process before it becomes a wearable fabric. Because of these attributes, it's hard to determine whether rayon is a synthetic or natural fabric; while its source material is organic, the process this organic material is subjected to is so strenuous that the result is essentially a synthetic substance.