Something that happens unexpectedly and by chance that is not inherent or natural to a given item or situation is adventitious, such as a root appearing in an unusual place on a plant.

Something that is apposite is relevant or suitable to what is happening or being discussed.

An auxiliary device is one that acts in support of a main one, such as a backup generator that replaces power in case the main source of that power goes down.

A cardinal rule or quality is considered to be the most important or basic in a set of rules or qualities.

Chaff is the husks of grain separated from the edible plant; it can also be useless matter in general that is cast aside.

A cohesive argument sticks together, working as a consistent, unified whole.

A constituent part of something makes up part of the whole thing.

Those things that constitute something form or make it up.

The crux of a problem or argument is the most important or difficult part of it that affects everything else.

If you cull items or information, you gather them from a number of different places in a selective manner.

A cynosure is an object that serves as the center of attention.

Detritus is the small pieces of waste that remain after something has been destroyed or used.

Something described as dross is of very poor quality and has little or no value.

Efficacy is the ability or power to produce an expected effect or result.

Someone who is feckless is incompetent and lacks the determination or skill to achieve much of anything at all in life.

If something is gratuitous, it is freely given; nevertheless, it is usually unnecessary and can be harmful or upsetting.

Something incidental refers to a minor occurrence that is related to something more important.

An indispensable item is absolutely necessary or essential—it cannot be done without.

Someone who is ineffectual at a task is useless or ineffective when attempting to do it.

An inherent characteristic is one that exists in a person at birth or in a thing naturally.

An innate quality of someone is present since birth or is a quality of something that is essential to it.

Something that is integral to something else is an essential or necessary part of it.

Two irreconcilable opinions or points of view are so opposed to each other that it is not possible to accept both of them or reach a settlement between them.

Irrelevant information is unrelated or unconnected to the situation at hand.

Something that is of paramount importance or significance is chief or supreme in those things.

The periphery of a place is its boundary or outer edge.

Something that is pivotal is more important than anything else in a situation or a system; consequently, it is the key to its success.

Something predominant is the most important or the most common thing in a group.

Something that is prolix, such as a lecture or speech, is excessively wordy; consequently, it can be tiresome to read or listen to.

Something that is succinct is clearly and briefly explained without using any unnecessary words.

Something that is superfluous is unnecessary; it is more than what is wanted or needed at the current time.

If you have a surfeit of something, you have much more than what you need.

When you are presented with a synoptic view of a written work or subject, you receive a summary or general overview of the entirety of its contents.

A tangential point in an argument merely touches upon something that is not really relevant to the conversation at hand; rather, it is a minor or unimportant point.

To be terse in speech is to be short and to the point, often in an abrupt manner that may seem unfriendly.

Something trivial is of little value or is not important.

If you truncate something, you make it shorter or quicker by removing or cutting off part of it.

Something that is verbose, such as a speech or article, contains too many words.

A watershed is a crucial event or turning point in either the history of a nation or the life of an individual that brings about a significant change.