A slide was an image displayed on a screen as part of a series.
In 2269, Spock questioned whether the slide showing Lieutenant Mira Romaine's brain circuitry pattern was the correct one, as it matched perfectly with the impulse tracking of the unknown aliens. As Nurse Christine Chapel had followed Romaine's testing step by step, Doctor Leonard McCoy felt there was no possibility of error. Upon asking the computer to compare the two, it confirmed they were indeed identical. (TOS: "The Lights of Zetar")