tl;dr: include the actual license. Just mentioning the license is not sufficient.
Many people seem to think that just saying “LICENSE: MIT” or “LICENSE: GPL” suffices. That is false.
MIT License (http://opensource.org/licenses/MIT) reads:
Permission is hereby granted, free of charge, to any person obtaining a copyof this software and associated documentation files (the “Software”), to dealin the Software without restriction, including without limitation the rightsto use, copy, modify, merge, publish, distribute, sublicense, and/or sellcopies of the Software, and to permit persons to whom the Software isfurnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included inall copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS ORIMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THEAUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHERLIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS INTHE SOFTWARE.
The license must be included in its entirety. Lazily saying MIT doesn’t cut it.
GPL Version 3 (http://opensource.org/licenses/gpl-3.0.html) reads
An interactive user interface displays “Appropriate Legal Notices” to the extent that it includes a convenient and prominently visible feature that (1) displays an appropriate copyright notice, and (2) tells the user that there is no warranty for the work (except to the extent that warranties are provided), that licensees may convey the work under this License, and how to view a copy of this License. If the interface presents a list of user commands or options, such as a menu, a prominent item in the list meets this criterion.
So you don’t need to include the entire license, but you do need to give instructions for viewing the license.
Apache License Version 2 (https://www.apache.org/licenses/LICENSE-2.0.html) reads
You may reproduce and distribute copies of the Work or Derivative Works thereof in any medium, with or without modifications, and in Source or Object form, provided that You meet the following conditions:
You must give any other recipients of the Work or Derivative Works a copy of this License; and
This is a bit more nebulous because, unlike GPL, this license does not explicitly allow you to merely link to the license. However, according to the appendix, it is acceptable to do so:
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
EDIT: There are most likely similar clauses in nearly every other commonly-used licenses. Per request, the ISC license (http://opensource.org/licenses/isc-license.txt) also states:
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.
(clearly, ISC has a similar requirement)