Substituent
In a structural diagram or formula, the letters R or X are used to represent a substituent group. An organic substituent such as methyl, ethyl, aryl can be written as R (or R1, R2, etc.)  This is a generic placeholder. The R derived from radical or rest. The R can represent as many atoms as the author wants. The first to use this symbol was Charles Frédéric Gerhardt in 1844.