The ISC License is a short permissive license written by the Internet Systems Consortium, the organization behind widely deployed infrastructure software such as the BIND DNS server. It was created as a deliberately stripped-down equivalent of the 2-clause BSD and MIT licenses, conveying the same permissions in slightly fewer words by removing language that had become legally redundant.
The entire permission grant is a single sentence. As published by the Open Source Initiative, it reads “Permission to use, copy, modify, and/or distribute this software for any purpose with or without fee is hereby granted, provided that the above copyright notice and this permission notice appear in all copies.” The only obligation is to preserve the copyright notice and the permission notice, exactly the attribution-only model shared by the other permissive licenses.
The rest of the text is a warranty disclaimer in capital letters, stating that “THE SOFTWARE IS PROVIDED “AS IS” AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE” and that the author shall not be liable for damages. There is no copyleft clause and no patent clause, so the ISC License is functionally interchangeable with the MIT and 2-clause BSD licenses for almost all purposes.
The phrase that makes ISC distinctive is “with or without fee.” Earlier permissive licenses sometimes separated permission to use without charge from permission to charge; ISC collapses both into one clause. The license also omits the older, now-unnecessary requirement that the disclaimer be reproduced in binary distributions, reflecting a view that such wording added length without legal effect.
ISC is the default license of the OpenBSD project, whose developers adopted it as the preferred license for new code, and it is the default suggested by the npm package manager when initializing a new JavaScript project. Those two roles have given it broad reach: a large share of packages in the npm registry carry the ISC identifier. The Software Package Data Exchange lists it under the identifier ISC, and because it imposes nothing beyond attribution it sits comfortably alongside MIT and BSD code in mixed-license projects.