The term 'Original Seven' refers to the first seven US astronauts selected for the Mercury project, which spearheaded US manned space flight. They emerged successful from a selection process that began in January 1959, when NASA began whittling down a list of several hundred possible candidates - all experienced military pilots - to a short list of 32. These 32 then underwent intensive medical and psychological testing, from which 18 'certified supermen' emerged. From these 18 the Original Seven were chosen, and presented at a press conference on April 2, 1959. They were Scott Carpenter (Navy), Gordon Cooper (Air Force); John Glenn (Marines), Virgil Grissom (Air Force), Walter Schirra and Alan Shepard (both Navy), and Donald Slayton (Air Force).
Reflecting the 'Seven' theme, the name of each Mercury capsule the astronauts flew in included the number 7. Shepard (Freedom 7) and Grissom (Liberty Bell 7) made the first two, suborbital, Mercury flights in 1961, while Glenn (Friendship 7), Carpenter (Aurora 7) , Schirra (Sigma 7) and Cooper (Faith 7) together spent a total of more than 44 hours in orbit in 1962/63.
Because of a suspected heart condition, Slayton missed out on the Mercury flights, and indeed the Gemini ones that followed. He had to wait until 1975 to make his space debut, on the ASTP mission.
