Use this tag when the software you're seeking should not require a fixed installation on a dedicated machine. For cross-platform software, please use the [cross-platform] tag.
For Windows, specifically, portable software will not alter the Windows registry or store any files outside of the installation folder (or its subfolders).
Note: On Software Recommendations SE, we do not use the tag "portable" to refer to cross-platform software. For cross-platform software, please use cross-platform.