why is rayon called as semisynthetic fibre?

Hi,

The synthetic fibers are those which are made by improving naturally occurring animal and plant fibers, 

Rayon was the first artificial (man-made) fibre. It is a manufactured from cellulose fibre. It is produced from naturally occurring polymers it is neither a truly synthetic fibre nor natural fiber.

Thus it is a semi synthetic fiber.

Hope it helps.

Cheers!