R-group
When a chemist sketches a molecule and writes a plain letter R hanging off it, the R is a placeholder — a slot that says "some group goes here, and we will try several." The R-group is exactly that variable piece attached to a fixed scaffold, the part the chemist deliberately changes from analog to analog.
More formally, an R-group is a substituent at a defined attachment point on a constant core structure; the symbol R (R1, R2, and so on for multiple slots) marks where variation occurs. Organizing a series this way — one or a few R-groups varied over a shared scaffold — is what makes SAR readable, because activity can be tabulated against each R-group and the responsible feature identified. This layout underlies R-group decomposition, the standard way medicinal-chemistry data is broken apart for analysis.
The notation is a tool for thinking, not a chemical fact: the same physical molecule can be carved into scaffold and R-groups in more than one way, and choosing a sensible decomposition matters. Treating an R-group as fully independent of the rest of the molecule can also mislead, because neighboring groups and the core can change what a given R-group does.
A spreadsheet listing one scaffold against R = H, F, Cl, OMe, CN with each row's IC50 is a classic R-group SAR table, read down the column to see which group wins.
Varying one R-group over a fixed core is the everyday format of SAR data.
R-group is a notation; the substituent itself is a real chemical group. The two ideas overlap, but substituent effect describes the chemistry while R-group describes the bookkeeping slot.