BSD refers to all sorts of things, but notably the version of UNIX from the University of California at Berkeley (the Berkeley Standard Distribution--or, according to Wikipedia  and rdt's memory, "... Software ...") and the license thereof. Tcl (which also started out at the University of California) uses a very similar license, though without the advertising clause (i.e., you are not required to put a big message in saying that you are using Tcl and acknowledging the original authors)."The State of the Demon Address"  aims to describe the different flavors of BSD Unix as of fall 2004.
rdt Hmmm, my recollection of BSD from the mid 1980's is that it was the Berkeley Software Distribution.
Kevin Kenny noted in the Tcl chatroom on 2003-06-12: "UCB removed the advertising clause in 1999."
LES I'd risk saying that, in the 21st century, whenever someone talks about BSD, they most probably mean either the BSD license or one of the three most popular "BSD UNIX" distributions: FreeBSD, NetBSD and/or OpenBSD.