Enterprise application integration (EAI) is the use of software and computer systems' architectural principles to integrate a set of enterprise computer applications.

Enterprise application integration is an integration framework composed of a collection of technologies and services which form a middleware or "middleware framework" to enable integration of systems and applications across an enterprise.

Many types of business software such as supply chain management applications, ERP systems, CRM applications for managing customers, business intelligence applications, payroll, and human resources systems typically cannot communicate with one another in order to share data or business rules. For this reason, such applications are sometimes referred to as islands of automation or information silos. This lack of communication leads to inefficiencies, wherein identical data are stored in multiple locations, or straightforward processes are unable to be automated.^{[citation needed]}

Enterprise application integration is the process of linking such applications within a single organization together in order to simplify and automate business processes to the greatest extent possible, while at the same time avoiding having to make sweeping changes to the existing applications or data structures. Applications can be linked either at the back-end via APIs or (seldom) the front-end (GUI).^{[citation needed]}

In the words of research firm Gartner: "[EAI is] the unrestricted sharing of data and business processes among any connected application or data sources in the enterprise."

The various systems that need to be linked together may reside on different operating systems, use different database solutions or computer languages, or different date and time formats, or could be legacy systems that are no longer supported by the vendor who originally created them. In some cases, such systems are dubbed "stovepipe systems" because they consist of components that have been jammed together in a way that makes it very hard to modify them in any way.^{[citation needed]}

If integration is applied without following a structured EAI approach, point-to-point connections grow across an organization. Dependencies are added on an impromptu basis, resulting in a complex structure that is difficult to maintain. This is commonly referred to as spaghetti, an allusion to the programming equivalent of spaghetti code.

For example, the number of connections needed to have fully meshed point-to-point connections, with n points, is given by ${\displaystyle {\tbinom {n}{2}}={\tfrac {n(n-1)}{2}}}$ (see binomial coefficient). Thus, for ten applications to be fully integrated point-to-point, ${\displaystyle {\tfrac {10\times 9}{2}}=45}$ point-to-point connections are needed, following a quadratic growth pattern.