Legacy software is software that has been around a long time and still fulfills a business need. It is mission critical and tied to a particular version of an operating system or hardware model that has gone end-of-life. Generally the lifespan of the hardware is shorter than that of the software. As time goes on, the hardware gets harder to maintain but is kept because it is installed and  working and has proven too complex or expensive to replace.

When SAP uses the term legacy, they mean any applications that were built by their customer. Or when that application is developed by a competitor. SAP used the term legacy to describe all CRM systems that are not SAP's CRM system.

   Legacy systems :-

Legacy systems include both the legacy software and the legacy hardware. Here at Stromasys, we virtualize the legacy hardware; we make no changes to the business software. Since the only change to the legacy system is replacement of the hardware with an industry-standard server, the arduous process of software certification does not have to be repeated.

Legacy systems are old software systems that are used by an organization. Usually, they rely on obsolete technology but are  essential to the business. It may not be cost-effective to rewrite or replace these systems and many organizations would like to use them in conjunction with more modern systems. One of the most important uses of services is to implement ‘wrappers’ for legacy systems that provide access to a system’s functions and data. These systems can then be accessed over the web and integrated with other applications.

To illustrate this, imagine that a large company maintains an inventory of its equipment and an associated database that keeps track of equipment maintenance and repairs. This keeps track of what maintenance requests have been made for different pieces of equipment, what regular maintenance is scheduled, when maintenance was carried out, how much time was spent on maintenance, etc. This legacy system was originally used to generate daily job lists for maintenance staff but, over time, new facilities have been added. These provide data about how much has been spent on maintenance for each piece of equipment and information to help to cost maintenance work to be carried out by external contractors. The system runs as a client-server system with special-purpose client software running on a PC.