A slide rule was a calculating tool used by scientists, engineers and navigators before handheld electronic calculators made them obsolete in the late twentieth century. Slide rules were used on Apollo missions for routine calculations.
By 2151, slide rules were considered ancient. In response to Malcolm Reed's request for a sextant, Trip Tucker responded in an equally disparaging manner by saying he left it with his slide rule. (ENT: "Shuttlepod One")