A human being, in the abstract, can be defined as a biochemical process -- and its corresponding systems, input, and output -- genetically distinct from, and incapable of genetic recombination with, processes and systems which meet the morphological and genetic criteria for "non-human." Technically, all physical objects to which symbols or concepts refer are interrelated, making their separation and definition arbitrary, but the concepts which are constructed from those objects can be given definite shape for the purposes of analysis and ideation. Words and concepts, therefore, can be given absolute definitions, because they are artificial in construction, while physical objects and other referents cannot undergo such objective abstraction (they are still necessarily abstracted by our sense organs, and we have no way of knowing what the true source of the abstractions is, however). Because of this, out of pure, practical necessity, we must give shape and constitution to any arbitrary set of objects or abstractions with which we interact, and human beings are no exception.
However, there are two erroneous ways to define a human:
1. Reducing him or her to one or several particular qualities or sets of qualities. Examples: Defining someone as "smart," "athletic," "fat," "black," "quiet," "artsy," "an atheist," "a liberal," etc. Obviously, this is problematic, because humans are complex organisms, and to reduce them to arbitrary facets of their so-called personalities is to gloss over essential nuance.
2. Assuming that the qualities which are currently applicable to him or her will always be applicable, or are applicable regardless of context. Examples: Defining someone as a creationist and consequently ignoring his or her attempts to have a philosophical discussion under the pretense that his or her beliefs are unshakable; defining someone as quiet after having interacted with him or her in only one kind of environment.
By all means, indicate where an "ism" applies to a person in the sense that it is something with which they agree (only if they universally agree with it, though), but refrain from indicating that the person is an "ist," and from any of the above. Tangentially, when it comes to "isms," it is important that you do not espouse any yourself, as it is impractical to invest in a belief, or to believe in anything at all; making probability assessments, then subsequently taking practical action to test the utility or efficacy of an idea -- all while never assuming that what you are acting as though you believe to be true actually is -- is the only way to live -- for now.
Also, many "isms" are bound by entirely independent qualities, making them pointlessly arbitrary and impractical. For the most part, discuss ideas individually; do not coin words for sets of ideas unless it is practical to do so, and above all else, where a proposed quality is not inherent in the definition of a predefined category, refrain from placing an idea sharing the quality into the category (unless it meets the actual criteria, of course).
Example illustrating the different kinds of isms: "Atheism" is simply the absence of a belief in a god, while "liberalism" contains so many concepts in its definition that it would be incredibly impractical to ever associate it with how you view the world.