Reciprocity means a mutual or cooperative interchange of favors or privileges. Something is reciprocal when it is performed, experienced, or felt by both sides. (The American Heritage Dictionary of the English Language, 4th edition.)
The GPL license is reciprocal, because it is “performed, experienced, or felt” by both sides—the licensor and the licensees both use the GPL.
In non-legal terms, I'd put this as "the GPL allows you to redistribute in kind: by extending the offer the original licensor made to others." The "price" of redistribution is agreeing to public participation in the co-evolution of the software.
This is not to be confused with Artifex's offer to dual-license: you may either accept the terms of the GPL or purchase a commercial license. These two worlds do not interact other than Artifex, the copyright holder, uses its rights under copyright law to offer these two alternatives.
