Something that is a standalone exists without requiring extras in order to operate. In the world of computers and the Internet, the basic definition of a standalone program is a program that functions as is when booted up or launched. Standalone software contrasts with a variety of other types of software, however, and there are several, somewhat contrasting concepts.
System software is the underlying software that runs on a computer. A computer application is made to run on one or more types of system software. System software is not considered to be standalone software, a term which generally is only used to apply to application software.
A plug-in is an add-on that enhances the working of a software application. It may be supplied by the manufacturer or created by third parties, and it may be freeware, shareware, or software for purchase. In any case, because it depends on the underlying software app in order to run, it is not an example of a standalone program, but the software it goes with may well be a standalone program. Modular software is generally a larger product than a plug-in, but it, too, attaches to an underlying base. If the base runs by itself, it may be standalone software, but if it requires one of the modules to run, it may not be, and the modules certainly are not.
An application suite is generally a group of standalone software grouped together for the purposes of marketing. Sometimes, in addition to standalone programs, an application suite will include software that serves to connect individual portions of the suite. If this software cannot be used on its own, without the other components of the suite, then it is not standalone software.
Another meaning of standalone software refers to the location from which it runs. There is some software that can run from a storage device without actually being installed on the computer. Software that does not require installation, but can still be run is another meaning given to the term standalone software.
Some programs cannot run without calling upon resources from the system, for example. A piece of software that runs without reference to the environment is another type of software that is referred to as a standalone program.
The term "standalone program" is very confusing, particularly in this day and age when a lot of applications have to have Internet access before they will run as intended. Almost all mobile apps are like that and there are more than a few computer programs that behave that way, too.