CKEditor is available as a commercial license at https://ckeditor.com/ or a choice of GPL/LGPL/MPL at https://github.com/ckeditor
This is the gist of its terms:
Licensed under the terms of any of the following licenses at your choice:
- GNU General Public License Version 2 or later (the "GPL")
- GNU Lesser General Public License Version 2.1 or later (the "LGPL")
- Mozilla Public License Version 1.1 or later (the "MPL")
You are not required to, but if you want to explicitly declare the license you have chosen to be bound to when using, reproducing, modifying and distributing this software, just include a text file titled "legal.txt" in your version of this software, indicating your license choice. In any case, your choice will not restrict any recipient of your version of this software to use, reproduce, modify and distribute this software under any of the above licenses.
And as @apsilers commented:
And for the OP's benefit: the weak copyleft options (LGPL and MPL) mean that distributions that include CKEditor as a library only need to include the source of CKEditor itself, not the entire application.