The BitTorrent Open Source Licence, is derived from the Jabber Open Source License, which is an Open Source Initiative (OSI) approved license. Former versions of the BitTorrent client (before 6.0) and related pieces of software are licensed under this License which is available here: http://www2.bittorrent.com/legal/bittorrent-open-source-license
A noteworthy aspect of the BitTorrent Open Source Licence is that it does not grant trademark licence. The trademark "BitTorrent" is owned by the company BitTorrent Inc. and is governed by the Trademark Use Guidelines available here: http://www.bittorrent.com/trademark.html
Open Source claims
Although the licence is derived from an OSI approved license, this licence hasn't been approved. Furthermore, the approved version of the Jabber licence is no longer used or recommended for use by its authors.
The Free Software Foundation considers it to be a free software license, albeit one incompatible with the GNU General Public License.