Today's word is reductionism. This word is often used in philosophy, especially amateur philosophy. It's important to recognize that there are at least four distinct meanings of this word in two two classes: describing something in terms of something else, and describing only a part of something.
To keep our definition grounded in objective reality, we will consider the case of waves in water, the sort that occur when we drop a rock in a pool of water.
Ontological Reductionism: We can describe a phenomenon in terms of something that really, concretely exists. For example, a wave in water actually is the positions and momenta of the various water molecules.
Explanatory Reductionism: We can describe something that really exists using fictional entities, entities that do not actually exist, for the purpose of convenience. For example, we can explain many features of waves in water using trigonometric equations, equations that are about the ratios of the sides of right triangles; no right triangles, however, actually exist anywhere in waves in water.
Idealistic Reductionism: We can focus on some particular aspect of some state of affairs, eliminating distracting side issues. For example, when we talk about waves in water, we are ignoring as mostly irrelevant the temperature, pressure, frictional, chemical characteristics, etc. of water.
Eliminative Reductionism: We assume that the properties we ignore don't actually exist, that we can always ignore them. Eliminative reductionism is sometimes not fallacious (for instance eliminative materialism in the philosophy of consciousness), but it often is fallacious, especially when it's confused with one of the other senses of reductionism.
(I'm talking about logical fallacies in general, and not about the validity or soundness of specific arguments, so I'm not going to provide specific arguments for rebuttal.)
I see a fair amount of confusion regarding reductionism in many philosophical (cough Postmodernism) discussions of science. I see philosophers fallaciously conflate ontological, explanatory or idealistic reductionism with eliminative reductionism: Just because I explain the gravitational behavior of a falling person without consulting his opinions or feelings in the matter does not mean that I consider his feelings to be universally irrelevant.
I also see a lot of confusion between ontological and explanatory reductionism. The charge is that it is often the case that ontological reductionism does not provide ease of explanation or prediction. For example, the ontological reductionism of waves in water to the positions and momenta of water molecules does not help us predict the phenomenon of waves: It is very difficult to compute the properties of waves by explicitly calculating the dynamic position and momentum of each individual water molecule.
But so what? Making predictions is only half the job of science: The other half is providing an overall picture of the world, and ontological reductionism is part of that half of the task, even if there's no immediate predictive benefit.
An especially pernicious use of fallacious eliminative reductionism is in ethical debate. It is often the case that some moral belief is predicated on several factors. An opponent will often reduce the justification to a single factor and attempt a reductio ad absurdum.
Conversations about abortion are an obvious case in point. My beliefs permitting abortion are predicated on three factors: (1) the non-sapience of an embryo, (2) the severe physical imposition an embryo places on the pregnant woman and (3) the embryo's lack of independent viability. Abortion opponents will often reduce this belief to only a single factor (usually (1)) and then claim that to be consistent, I ought to support infanticide, since infants are not in fact sapient. This is a fallacy of eliminative reductionism, since it unjustifiably ignores the additional predicates underlying my belief.
The fallacy of eliminative reductionism also creeps in by noting that my beliefs about abortion are not apparently predicated on speciesism. However, to say that the lack of speciesism underlying my position on abortion entails that I'm being inconsistent in, for instance, my practice of eating meat is fallacious. This charge ignores that since both an embryo and the mother have human genetics (and this is always the case regarding abortion), the speciesism is canceled out.
Abortion proponents sometimes commit the same sort of fallacy: For instance, it would be fallacious to argue that the criterion of substantial genetic independence establishes the separate existence of the embryo thereby commits one to considering twins to be a single organism (or one twin as superfluous).