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A reversed C in a circle, standing for permissions passed on.
Also known as: Reversed copyright symbol
A backwards C in a circle stands for copyleft — the practice of licensing a work so that anything built on it must carry the same freedoms. It has no legal force of its own and is not registered anywhere; the force comes from whichever licence the work actually uses.
Stands for copyleft: licensing a work so that derivative works must be released under the same terms, keeping the permissions attached as the work is reused.
Contexts: Software
Carries no legal effect on its own and is not a registered mark. A work is copyleft because of the licence it carries, not because of this symbol, which is a statement of intent rather than an instrument.
Contexts: Software
The term and the mirrored mark come from free software practice in the 1980s, as a deliberate inversion of all-rights-reserved copyright.
Contexts: Software
A circle outline enclosing a capital letter C reversed left to right, so its opening faces left rather than right.
The share-alike condition is the licensing term that actually imposes copyleft; this symbol only names the idea.
Both are built from circles and letter like forms, and both are used in software contexts.