mpv is a fork of mplayer2, which is a fork of MPlayer.
mpv as a whole is licensed as GPL version 2 or later (see LICENSE). Most source
files are GPLv2+, but some files are available under a more liberal license,
such as LGPLv2.1+, BSD, MIT, ISC, and possibly others. Look at the copyright
header of each source file, and grep the sources for "Copyright" if you need
to know details. Files without Copyright notice are licensed as LGPLv2.1+.
All new contributions must be LGPLv2.1+ licensed, or if the changes are done on
GPL code, must come with the implicit agreement that the project can relicense
the code to LGPLv2.1+ at a later point without asking the contributor. (This
is a safeguard for making potential relicensing of the project to LGPLv2.1+
easier.) Using a more liberal license compatible to LGPLv2.1+ is also ok.
Some GPLv3 or LGPLv3 code in mpv can be enabled by building with --enable-gpl3,
and in this case, the resulting binaries must be distributed under GPLv3.
For information about authors and contributors, consult the git log, which
contains the complete SVN and CVS history as well.
Note that mplayer2 as a whole is licensed under GPLv3+. This is because it uses
a copy of talloc (part of Samba), which is LGPLv3+, and the next compatible
license for this mix is GPLv3+.
MPlayer as a whole is licensed under GPLv2 (incompatible to GPLv3!), because
some files are licensed to GPLv2 (and _not_ any later version of the license).
In particular, this affects the file libmpdemux/demux_ty_osd.c. It is disabled
under mplayer2, and has been removed from mpv.
"v2.1+" in this context means "version 2.1 or later".