The Free On-line Dictionary of Computing (30 December 2018):

PDP-7

    A minicomputer sold by DEC in 1964.  It had a
   memory cycle time of 1.75 microseconds and add time of 4
   microseconds.  I/O included a keyboard, printer, paper-tape
   and dual transport DECtape drives (type 555).

   DEC provided an "advanced" Fortran II compiler, a Symbolic
   Assembler, Editor, DDT Debugging System, Maintenance
   routines and a library of arithmetic, utility and programming
   aids developed on the program-compatible PDP-4.

   [DEC sales brochure].

   The PDP-7 was considered reliable enough (when properly
   programmed) to be used for control of nuclear reactors and
   such.

   Around 1970 Ken Thompson built the operating system that
   became Unix on a scavenged PDP-7 so he could play a
   descendant of the SPACEWAR game.

   (1995-03-10)