What is aburdism?

Good question, Biller--it might elevate this forum to a higher level.
As I see it, "absurdism" and "nihilism" are virtual synomyms. They refer to the inherent subjectivity--or better said: intersubjectivity/cultural nature of life's meanings. The desire for universal, absolute and objective meaning is a religious orientation; I refer to religion in the worst sense of the term.
Nietzsche's pronouncement of the death of god was an annoucement that humanity had, with the end of the Church's hegemony over morality, launched humanity into a realm of existential responsibility. Our life is our creation, not that of a fictitious Creator.

From either a denial of "life" in general having objective (or any) meaning to only denying select features of it (one can include epistemological nihilism, etc., as knowledge and the rest are a part of life). But since life is found in conjunction with consciousness or is part of the subjective/objective contents of experience and thought, it seems to require a degree of pretense or forgetfulness to contend that it lacks meaning or global meaning at all.

The "objective world" of outer perception is independent of one's own personal will (that subjective, personal half of the dichotomy), but not the properties and rules of consciousness/experience in general, as shared by humans. An environment that is presented with discriminated objects/events and their spatial relations, and is thought about and theorized about afterwards, is just that: A world of understanding, a world that is understood -- if not in a wholly static or immutable manner. Thus it and the lives that are part of it can't avoid having meaning (this objective empirical exhibition even supports a story of itself, via brain science, of being a processed or conditioned product; it's not limited to being a conclusion of philosophical traditions anymore).

It's practical in some research fields and doctrines to treat the perceived external environment's apparent indifference to human interests and goals as teleological absence and absence of other brands of significance.

A "true" non-conscious circumstance which is supposed out of anti-panpsychic necessity/preference to be the cause of the objective world displayed and interpreted in consciousness (as well as one's individual subjectivity) does not even seem to garner complete insignificance, since -- after all -- it would also be yielding the aforementioned conditioned world with its interrelationships between things (i.e., it is significant in that regard). But a possible pure integration with or dissolving back into that non-conscious "stratum" is only before life and at death, which by virtue of being a "non-conscious" circumstance is a lack of evidence of everything as experience and reason (which of course would include "non-consciousness" lacking evidence for its own being, or evidence for this state of manifestation and understanding being absent).

What I'm trying to say is that meaning is not given (objective) it's artifactual (culturally constituted and socially constructed) . Of course most of my meanings are inherited (and often modified by me) in the complex biographical process of enculturation. But my species and society invented the meanings that I share--with some people and not with others.

So, nihilism is a denial of certain features of a given subject?

It can be characterized as more specific formulations (political nihilism, ethical nihilism, etc.) in addition to the broad annihilation or extreme skepticism of all values, concepts, loyalties, authority, and purposes. For instance, Daniel Dennett might be a qualia or phenomenal nihilist, as an alternative if indeed he resists being classed as an eliminative materialist. Friedrich Jacobi introduced the term nihilism and triggered the initial discourse among philosophers about it, employing it as a pejorative assessment / consequence of the Enlightenment. But there are surely instances of similar views (minus the label) going back to pre-Socratic philosophers.

To me:
Nihilism is the absence of absolutes in the realm of human meaning. When Nietzsche's proclamation of the death of God was metaphorical fanfare for the birth of epistemological nihilism, the announcement that we must create our own meanings--indeed the claim that that is what we have always done, even when we say our meanings (like Plato's ideals) are made by divine artificers.

Chaos is what appears to us when we cannot construct a meaning for some phenomenon that does not permit prediction and/or control. "Knowledge" is its opposite, that which enables prediction and control. With the new physics this is more problematic regarding the goal of control.

Anarchy is the absence of government and law.

This reminds me of what people know about the human psyche. I also want to add there is a difference between being in control or having the illusion of control. (Sorry, this is way off topic)

I agree. The feeling (or illusion) of control is to some extent what culture is about, a collective effort to ward off the chaos of Reality.
But I feel ultimately that that that feared Chaos is our very nature and therefore a kind of Order--relatively speaking.

Was Nietzsche's nihilism derived from his perspectivism, or vice versa, some other relationship between them, or no association at all?

I see his nihilism and perspectivism as entailments of each other.

Is absurdism by camus the belief that life has no universal meaning or no objective meaning but that it still has subjective meaning or the meaning you want to give? Is this correct of absurdism?

That's true of both Absurdism and Existentialism. As Kierkegaard wrote: "Truth is subjectivity".
The difference is the attitude between the two. For the existentialists, it's a good thing, an imperative thing, a necessary thing to give subjective meaning (with the help of God)

For absurdists, giving subjective meaning is a trivial matter. It's not necessary, it's not a good thing (or a bad thing), it's beside the point.

That's also the difference between Camus and Kierkegaard. And why Camus doesn't think a leap of faith towards a meaning provided by God is all that.

The way I think of it is that we seek meaning and purpose, because it is our inherent nature to do so. The universe, however, reveals none. Being an absurdist is to be true to your nature, and create a bubble of purpose and meaning for yourself, while simultaneously recognizing that it is your own construction, and therefore not take yourself too seriously.

There is no such thing as true nihilism if one has pre-conceived ideas about anything at all one way or the other prior to ones imaginary cancelling out of reasoning/meaning.

One is merely a + or - Nihilist in this instance….or a deluded Nihilist if one needs to be blunt and get to the point.