Unified Messaging (or UM) is the integration of different electronic messaging and communications media (e-mail, SMS, Fax, voicemail, video messaging, etc.) technologies into a single interface, accessible from a variety of different devices. While traditional communications systems delivered messages into several different types of stores such as voicemail systems, e-mail servers, and stand-alone fax machines, with Unified Messaging all types of messages are stored in one system. Voicemail messages, for example, can be delivered directly into the user's inbox and played either through a headset or the computer's speaker. This simplifies the user's experience (only one place to check for messages) and can offer new options for workflow such as appending notes or documents to forwarded voicemails.
Unified Messaging was expected by many in the consumer telecommunications industry to be a popular product, first augmenting and eventually replacing voicemail. However, UM was slow to gain consumer acceptance, and UM vendors such as Comverse were badly hit when the slowdown in the telecommunications industry in 2001 made carriers wary of spending large amounts of money on technology with little proven consumer demand.
Today, UM solutions are increasingly accepted in the corporate environment. The aim of deploying UM solutions generally is to enhance and improve business productivity while decreasing communication issues. UM solutions targeting professional end-user customers integrate communications processes into the existing IT infrastructure, i. e. into CRM, ERP and mail systems (e. g. CallXpress, Phoenixnet PH, Microsoft Exchange, Lotus Notes, SAP, etc.)
Indistinct Definitions 
Unified Messaging is an indistinct term that can refer to the typical definition of simple inclusion of incoming faxes and voice-mail in one's email inbox, all the way to dictating a message into a cell phone and the intelligent delivery of that message to the intended recipient in a variety of possible formats like text email, fax, or voice recording. Because of the nebulous definition of UM, it was number one on the 1998 Wired Magazine "Hype List". To bring the terminology up to date it is important now to recognize that UM incorporates voice as a key component of the offering. All these technologies need to be brought together: voice, instant messaging, email, voicemail, and web/voice/video conferencing. The aim is to make communication seamless, no matter what the method. The missing piece not mentioned that allows these technologies to be delivered together is 'presence'. If friends and colleagues are 'aware' of each other's state (e.g., available, busy, in a meeting, or offline), they can then choose the most appropriate communication method to use and therefore ensure effective communication.
Unified Messaging is not to be mistaken for Unified communications, although the two share some similarities. Hosted UC services lack real-time contacts like IM and presence-based telephone connections because those features are considered “frilly features” not demanded by the majority of hosted SMB customers.  So while Unified Messaging can be included in Unified communications, not all Unified communication services are related to Unified Messaging.
See also 
- "Unified Messaging. Definition and Overview". International Engineering Consortium. 2005. Retrieved 2009-04-02.[dead link]
- Kostek, Jessica (2009-02-29). "Frost & Sullivan: Unified Messaging to Become Ubiquitous Enterprise Communication Solution". TMCnet.com. Retrieved 2009-04-07.
- Popova, Elka (2007-07-05). "Customers Attest to the Value of Flexible Independent Messaging Solutions". Frost & Sullivan. Retrieved 2009-05-12.
- PC Magazine Unified Messaging definition The Computer Language Company Inc.
- 6:06 Hype List Wired.com
- Art Rosenberg and UCStrategies, "Don't Miss the 'Unified Messaging' Boat in UC