system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is "irreducible complexity" in regards to the theory of evolution?

I've read a few things but still don't think I get it.  

1.  What does the term originate and what does it mean?
2.  Why do creationists use this as "proof" against evolution?
gpt
One of the main ideas of evolution is that small changes add up, resulting in big adaptive changes over time. For instance, fish using fins to crawl across land to new mud puddles, fins get bigger, blablabla legs. 

Irreducible complexity is saying, "Well how would a creature incrementally evolve a wing? Half a wing won't help you fly, so it never would have been selected for in the first place. And what about an eye? 'Half' an eye is completely useless, so obviously a creature born with half an eye would not be selected for by evolution."

So, its saying that things like eyes and wings are "irreducible complexity." That is, they cannot be any 'less' and still have function. The conclusion being that incremental changes, as theorized by evolution, would never result in their existence.