Xapian is a free and open-source probabilistic information retrieval library, released under the GNU General Public License (GPL).

It is written in C++, with bindings to allow use from Perl, Python (2 and 3), PHP (5 and 7), Java, Tcl, C#, Ruby, Lua, Erlang, Node.js and R.[1][3] Xapian is highly portable and runs on Linux, OS X, FreeBSD, NetBSD, OpenBSD, Solaris, HP-UX, AIX, Windows, OS/2[4][2] and Hurd,[5][6] as well as Tru64.

[citation needed] Xapian grew out of the Muscat search engine, written by Dr. Martin F. Porter at the University of Cambridge.

[8] Xapian allows developers to add advanced indexing and search facilities to their own applications.

Organisations and projects using Xapian include the Library of the University of Cologne, Debian, Die Zeit, MoinMoin, and One Laptop per Child.