Apollo 18 was the first of the three final Apollo space missions that were cancelled by NASA in 1970. To be completely precise, the mission originally designated Apollo 18 did take place under the name Apollo 17 following the cancellation of the original Apollo 15 mission and subsequent renumbering. After the successful moon missions and in the wake of the near-disaster of Apollo 13 the last three flights in the Apollo programme were prudently budgeted out of existence before something went really wrong. After all, NASA had made its point, won the race to the moon and allowed the United States' space programme to assert itself after a decade of Soviet leadership. Apollo 18's mission would have been to land in Copernicus Crater on the moon.
A mission often called Apollo 18 was really the Apollo-Soyuz Test Project in which the first docking of Soviet and American spacecraft occurred. The American craft that took part in the project was an Apollo-type spacecraft nearly identical to the ones used in the lunar, and later in the Skylab, missions. That flight though, not being a lunar mission, was not part of the official Apollo sequence although Vance Brand, one of the astronauts expected to be part of the crew on the cancelled Apollo 18 mission, did take part.