BSD refers to all sorts of things, but notably the version of UNIX from the University of California at Berkeley (the Berkeley Software Distribution), the original license thereof and its many derivatives. Tcl, which also started out at the University of California, uses a very similar license, though without the so-called 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).
See the Wikipedia article for an overview of BSD-derived licenses.
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.