PRO BONO definition :

Pro bono publico (usually shortened to pro bono) is a phrase derived from Latin meaning ”for the public good”. The term is generally used to describe professional work undertaken voluntarily and without payment as a public service. It is common in the legal profession and is increasingly seen in marketing, technology, and strategy consulting firms. Pro bono service, unlike traditional volunteerism, uses the specific skills of professionals to provide services to those who are unable to afford them.
Pro bono publico is also used in the United Kingdom to describe the central motivation of large organizations such as the National Health Service, and various other NGOs, which exist ”for the public good”, rather than for shareholder profit.

more here:

http://en.wikipedia.org/wiki/Pro_bono