The Jargon File (version 4.4.7, 29 Dec 2003):

PDP-11


    Possibly the single most successful minicomputer design in history, a
    favorite of hackers for many years, and the first major Unix machine, The
    first PDP-11s (the 11/15 and 11/20) shipped in 1970 from DEC; the last
    (11/93 and 11/94) in 1990. Along the way, the 11 gave birth to the VAX,
    strongly influenced the design of microprocessors such as the Motorola 6800
    and Intel 386, and left a permanent imprint on the C language (which has an
    odd preference for octal embedded in its syntax because of the way PDP-11
    machine instructions were formatted). There is a history site.


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

PDP-11

   Programmed Data Processor model 11.

   A series of minicomputers based on an instruction set
   designed by C. Gordon Bell at DEC in the early 1970s (late
   60s?).  The PDP-11 family, which came after, but was not
   derived from, the PDP-10, was the most successful computer
   of its time until it was itself succeeded by the VAX.

   Models included the 11/23 and 11/24 (based on the F11
   chipset); 11/44, 11/04, 11/34, 11/05, 11/10, 11/15, 11/20,
   11/35, 11/40, 11/45, 11/70, 11/60 (MSI and SSI); LSI-11/2
   and LSI-11 (LSI-11 chipset).  In addition there were the 11/8x
   (J11 chipset) and SBC-11/21 (T11 chip) and then there was
   compatibility mode in the early VAX processors.

   The B and C languages were both used initially to
   implement Unix on the PDP-11.  The microprocessor design
   tradition owes a heavy debt to the PDP-11 instruction set.

   See also SEX.

   (1994-12-21)