Magnification was the process of enlarging the perceived size of an object for closer observation. This was a feature on viewscreens.
In 2266, after Spock detected a blip on the motion sensor of the USS Enterprise, Captain James T. Kirk ordered to go to full magnification on the viewscreen. (TOS: "Balance of Terror")
In 2268, after the Enterprise discovered indications of energy turbulence and encountered the space amoeba, Captain Kirk ordered a magnification factor three on-screen. (TOS: "The Immunity Syndrome")
In 2366, sensors aboard the USS Enterprise-D indicated the approach of a Borg vessel but there was no visual contact. Captain Jean-Luc Picard ordered Worf to use their current screen magnification to view fifty degrees starboard. Later, while examining a cross section of computer core processor 451, Geordi La Forge requested a magnification, factor fifty. Data later requested a magnification factor of 1,000 when viewing element 0299 of the same computer core processor. (TNG: "Evolution")