R-atom: A special type of atom that connects to a base molecule as ligand and abbreviates one or more possible structures. As there can be more than one R-atom in the same structure, they can be marked with R1, R2, R3, etc.
Root molecule or scaffold: The molecule to which the R-atoms connects. Note the R-atoms are part of of the root molecule as well.
R-group defintion: Lists of ligands that R-atoms abbreviate.
R-group: The whole structure described above. That is the root molecule and the R-group definition.