tobacco mosaic virus: the widely studied plant virus that causes tobacco mosaic; it was the first virus discovered (1892)
plant virus: a plant pathogen that is a virus consisting of a single strand of RNA
dead metaphor: a metaphor that has occurred so often that it has become a new meaning of the expression (e.g., `he is a snake' may once have been a metaphor but after years of use it has died and become a new sense of the word `snake')