The language of Shakespeare is fine for detailing the angst of young Danish princes and the murderous ambition of Scottish generals. It has stood up to the mad demands of Keats and Byron, the somnolent, metronomic precision of Thomas Penfield Jackson. But English has let down Bob Young, Richard Stallman and me. Specifically, it's the word free that has proved so reliably troublesome. In the open-source community, two of the definitions are singled out for special attention.
Free beer is the simpler concept.
In this context, free refers to the price. Free software is software that costs nothing. Public domain software falls into this category. The free software promoted by Stallman's Free Software Foundation is free as specified and guaranteed by the GNU Public License. Because Bell Labs makes Plan 9 available for download at no cost, the operating system is free software in the free-beer sense.