Custom software is a software that is specially developed for some specific organization or other user. As such, it can be contrasted with the use of software packages developed for the mass market, such as commercial off-the-shelf (COTS) software, or existing free software.

Since custom software is developed for a single customer it can accommodate that customer’s particular preferences and expectations. Custom software may be developed in an iterative processes, allowing all nuances and possible hidden risks to be taken into account, including issues which were not mentioned in the original requirement