A software and/or hardware landscape dominated by a single architecture, brand, or even product. Common examples are the dominance of MicrosoftCorporation software in many companies and CiscoSystems hardware on the InterNet.
It is considered a serious risk to run a MonoCulture, since a single security (or other) flaw in (any part of) the system you're running will expose your entire infrastructure to harm. Obviously, removing that flaw will then also require touching your entire infrastructure. An example that has probably affected everyone is the Microsoft Outlook/OutlookExpress MonoCulture, which is the biggest factor in the spread rate of email worms (often mistakenly referred to as viruses).