A patch is a software update comprised code inserted (or patched) into the code of an executable program. Typically, a patch is installed into an existing software program. Patches are often temporary fixes between full releases of a software package.
Patches may do any of the following:
Fix a software bug
Install new drivers
Address new security vulnerabilities
Address software stability issues
Upgrade the software
Software patches can be free or available for sale. Some companies deliver patches to registered users only. Patches are usually available as Internet downloads.
If the original source code is proprietary and not released to the general public, then patches are released as executable binary code. Patches alter the existing programming code by modifying it or replacing it completely.
Patches have become extremely important as a methodology for updating programs or new system security threats which appear regularly, especially in online environments. Formerly patches were installed manually. Today automatic updates are very popular and are available as self-installing packages from the software vendors support pages at their website.
Although patches can vary in size from several kilobytes to hundreds of megabytes, patches are usually perceived as being rather small. Common to Microsoft Windows operating system large patches are generally named service packs, and can be over 100Mb.
Source: https_www_techopedia_com/definition/24537/patch