Viroid

Viroids are plant pathogens that consist of a short stretch (a few hundred nucleobases) of highly complementary, circular, single-stranded RNA without the protein coat that is typical for viruses. The smallest so far is a 220 nucleobase scRNA (small cytoplasmic RNA) associated with the rice yellow mottle sobemovirus (RYMV). In comparison, the genome of the smallest known viruses capable of causing an infection by themselves are around 2 kilobases in size.
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Vi·roid n. An infectious particle, smaller than a virus, that consists solely of a strand of RNA and is capable of causing disease in plants. [more]Viroid - Community and Resources

Noun
1. the smallest of viruses; a plant virus with its RNA arranged in a circular chromosome without a protein coat
(synonym) virusoid
(hypernym) plant virus

A disease causing agent consisting of only a single-stranded, short (270 to 380 nucleotides) RNA molecule.