1. The cultivation of a single crop on a farm or in a region or country.
2. A single, homogeneous culture without diversity or dissension.
In biology natural monocultures are not unknown, but tend to be rare as they can be overwhelmed by a variety of biological, climatelogical and other natural events. In agribusiness artificial monocultures are common as they tend to be efficient and profitable. In forestry one can create "managed forests" which tend to produce timber of consistent size and type maximizing profit per acre. The same goes for wheat and corn.
Unfortunately agri-moncultures tend to be vulnerable to attack with heavy pesticide use being necessary to allow their existence. There are also risks that you can invite disaster by crop failure and this is a major worry in among biologists - particularly those who worry about food production.
The problem with monocultures is that they are extremely sensitive to attack. Monocultures consist of identical plants with identical defenses. Unlike a diverse stand of plants, a disease or infestation can devastate a monoculture, rendering the entire forest or fields worthless. In a heteroculture, diseases and infestations can be stopped when they don't have an nearby host to attack - in a monoculture, every adjacent plant is an inviting host, waiting for an attacker to get lucky.
More than ninety percent of the world's computers are based on Microsoft Windows with the numbers in many corporations approaching 100%. Products like Office, Outlook, Enchange and Explorer enjoy equally large penetrations. To first order each of our computers has a nearly identical genetic makeup in hardware and software.
This standardization has allowed for reductions in cost of hardware, perhaps in software (although that is debatable) and in training. Unfortunately it has turned our society into a computational monoculture with many critical elements of society - government, finance, police, fire, military, transportation, healthcare, etc etc being made vulnerable to the threats that monocultures face.
Even if operating systems and applications packages were equally secure, the version that enjoys monopoly is inherently more vulnerable - studies in the biological world indicate that this vulnerability is non-linear -- extrapolating to the networked world, a system that has 5% penetration is much safer than a simple ratio (95/5) would suggest.
So for those of you who are battling issues associated with Windows, note that the attacks can and will get more serious and this is part of the cost of ownership. In the end going with Windows is somewhere on the continuum between being much more cost effective than using less dominant systems to your going out of business and the call is a business decision for CIOs and CTOs.
For systems that control the basic underpinnings of society the potential effects are so dangerous that it is indefensible to rely on a monoculture.
If you are a home user or small organization with only a few computers, using something other than the dominant software and OS will give you enormous protection. If you have more machines and or a robustness requirement, you may wish to consider a mix of operating systems (nothing that the safest mix would exclude the dominant OS). Publicly held businesses should be required to show how diverse their infrastructures are (perhaps on their Form 10-Qs) and let investors decide where their investments should be made.
This is not Microsoft bashing - if Linux or OS X were at the number one position, and if they were as secure/insecure as Windows - the same would apply to them.
Our local school system has lost enough due to worms and viruses on Windows to more than make up the difference in cost between the Dell/XP platform they are using and and equal number of iMacs with OS X. As a tax payer I'm frustrated.