A series of large, 36-bit mainframe-like systems built by DEC; they were basically a re-implementation of the earlier PDP-6 architecture, whose engineering had been a failure. (The machines were so similar at the programming level that PDP-6 code could run on a PDP-10.)
DEC sold 4 different generations of PDP-10 processors: the KA10, the KI10, the KL10, and the KS10. The first three were marketed as the DECsystem-10, running the TOPS-10 operating system; the third was also sold as the DECSYSTEM-20, running TOPS-20. (The varying capitalization was the result of a trademark infringment suit.)
PDP-10s were very important machines on the early Internet, being one of the few (relatively!) cheaply available machines which could run a full NCP and later TCP/IP stack as a multi-user environment at the time.