Under the International Code of Botanical Nomenclature (ICBN) Rosidae is a botanical name at the rank of subclass. Circumscription of the subclass will vary with the taxonomic system being used; the only requirement being that it includes the family Rosaceae.
Under Phylocode Rosidae is a clade defined as the most inclusive crown clade containing Rosa cinnamomea but not Berberidopsis corallina nor Dillenia indica nor Gunnera manicata nor Helianthus annuus nor Saxifraga mertensiana nor Stellaria media nor Viscum album. 
A well-known usage of the former sense is in the Cronquist system; in the 1981, original, version of that system the circumscription was:
- subclass Rosidae
The Phylocode definition includes Crossosomatales, Geraniales, Myrtales, Fabidae (Celastrales, Cucurbitales, Fabales, Fagales, Huaceae, Oxalidales, Malpighiales, Rosales and Zygophyllales), Malvidae (Brassicales, Dipentodontaceae, Gerrardinaceae, Malvales, Sapindales, and Tapisciaceae), "probably" Picramniaceae, and "possibly" Vitaceae.
There is considerable overlap between two definitions (some apparent differences are the result of the use of more broadly drawn orders in the second), but Apiales, Cornales, Proteales and Santales and parts of Rafflesiales are excluded from the second, and many groups from Cronquist's Hamamelidae and Dillenidae are included.
In both senses the term rosid applies, as an adjective and noun, to members of the group. In the APG and APG II systems, which eschewed formal formal botanical names between the ranks of class and order the term rosids is used to define an informal clade corresponding to the PhyloCode (Cantino et al) sense.
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