Ask HN: Define “Cultural Fit”

In the "How Can We Achieve Age Diversity in Silicon Valley?" [1] thread, the term "cultural fit" ended up coming up as one potential source of age discrimination, and the thought occured to me: What do we mean exactly when we say "cultural fit"?

If I asked you out of the blue to define as carefully as you can what you mean by "cultural fit", in the context of hiring, what would you say?

[1]:https://news.ycombinator.com/item?id=10412284 , for posterity