Analog Product Design
The very first designs that came about were analog designs, a Vacuum Tube amplifier is an example. Analog is the representation of data or physical parameter in an accurate and precise decimal number like + 72.182 mV. The greater the resolution required, the more demanding is the design for achieving high accuracy and low drift.
Analog Circuits - Opamps, Comparators, Mixed Design Circuits.
As an analogy, see the conventional water tap as an analog control, an electric light ON-OFF switch as a digital control. After the handheld Abacus calculator (low-power, durable, bio-degradable), Mechanical and electro-mechanical computers were made and studied, The very first electronic computers were Analog Computers using an opamp as a building block.