Small, linear single-stranded RNA molecules functionally acting as molecular parasites of certain RNA plant viruses. Satellite RNAs exhibit four characteristic traits: (1) they require helper viruses to replicate; (2) they are unnecessary for the replication of helper viruses; (3) they are encapsidated in the coat protein of the helper virus; (4) they have no extensive sequence homology to the helper virus. Thus they differ from satellite viruses which encode their own coat protein, and from the genomic RNA; (=rna, viral); of satellite viruses. (From Maramorosch, viroids and Satellites, 1991, p143)