Programmed Data Processor model 10.
The series of mainframes from DEC that made time-sharing real. It looms large in hacker folklore because of its adoption in the mid-1970s by many university computing facilities and research labs, including the MIT AI Lab, Stanford, and CMU. Some aspects of the instruction set (most notably the bit-field instructions) are still considered unsurpassed.
The PDP-10 was eventually eclipsed by the VAX machines (descendants of the PDP-11) when DEC recognised that the PDP-10 and VAX product lines were competing with each other and decided to concentrate its software development effort on the more profitable VAX. The machine was finally dropped from DEC's line in 1983, following the failure of the Jupiter Project at DEC to build a viable new model. (Some attempts by other companies to market clones came to nothing; see Foonly and Mars.) This event spelled the doom of ITS and the technical cultures that had spawned the original Jargon File, but by mid-1991 it had become something of a badge of honourable old-timerhood among hackers to have cut one's teeth on a PDP-10.
See TOPS-10, AOS, BLT, DDT, DPB, EXCH, HAKMEM, JFCL, LDB, pop, push.