Microfiche
A flat sheet of microfilm the size of an index card, holding a grid of heavily reduced page images read in a magnifying viewer.
A microfiche is about 105 × 148 mm and typically holds dozens to a hundred pages in a grid, reduced around 24 times. You read it in a viewer that back-lights and magnifies one frame at a time. Libraries, archives and parts catalogues traded shelf-metres for a box of cards, and the film lasts for decades if it is stored properly.
Computers could write straight to fiche through COM (Computer Output Microfilm), which replaced stacks of green-bar line-printer paper with a handful of sheets. Roll microfilm and the aperture card are its closest relatives.