To be more specific, a microcosm is either
a small model or representation of something much larger
or something that's complex enough to be its own little self-contained world.
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Part of speech:
(Countable nouns, like "bottle," "piece," and "decision," are words for things that can be broken into exact units. You talk about "a bottle," "three pieces," and "many decisions."
Likewise, talk about one microcosm or multiple microcosms.)
An alternate to "microcosm" is "microcosmos."
The plural is "microcosms."
For the adjective, you have your pick of "microcosmal," "microcosmic," and "microcosmical."
And the adverb is "microcosmically."
How to use it:
When you need to emphasize the incredible complexity of something small, or when you need to emphasize the striking way in which something small represents something larger, call it a microcosm.
Things we often call microcosms include streets, neighborhoods, communities, stores, schools, and churches, as well as more abstract things like stories, events, actions, decisions, competitions, conversations, conflicts, etc.
Often we say that one smaller thing is a microcosm of the larger thing: a neighborhood that's a microcosm of a culture, a corporation that's a microcosm of ruthless capitalism, an incident that's a microcosm of America's racial inequality.
Lastly, you can use the phrase "in microcosm" to mean "on a very small scale:" we see or understand something in microcosm, something appears in microcosm, something is something else in microcosm, something captures or presents something else in microcosm, and so on.
examples: "Last Resort," the first single from Papa Roach's 2000 album Infest, is a microcosm of alternative metal: a cry for help, an angry shout, a violent explosion of hyperbole.
The crowded cafe is hot and humid, with the occasional relief of an ocean breeze, and amid the tables float the warm scents of rice and Spam and the sound of chatter in English, Japanese, and Hawaiian. It's Hilo in microcosm.