A series of graphing calculators made by Hewlett Packard. Including the 48G itself, and the HP 48GX. The former featured 32K of RAM, and the latter 128K.
Unlike most other graphing calculators, such as those from Texas Instruments, the 48G series featured Reverse Polish Notation, or RPN. Although RPN can be tricky to master at first, it is generally considered by it's supporters to be a great deal more efficient to use than traditional modes of calculation.
Features include 3D graphing capability, an equation editor, a matrix editor, an equation solver, unit conversion and an extensive library of commonly used physics and engineering equations and constants.
Calculators in the 48G series also feature communications (both with other calculators and with other devices such as computers or printers) via either IR or a serial cable.