Definition: A pitch is a writer's description of a potential story to an editor. A pitch can be delivered verbally -- if you're on staff pitching to your editor -- or sent via email.
So what does a pitch entail? A pitch lays out why a story matters. A pitch essentially makes the case for doing a certain story at a certain point in time. And a good pitch letter should quickly and succinctly do a few things -- introduce the author, summarize the story the author wants to write and explain why that story matters. A good pitch should also make the case for why its author is the best person to write that story.
A good pitch is the key to landing an assignment. For more on the art of pitching, check out Mediabistro's "How to Pitch" series.
Examples:
Even though the editor had never worked with this particular writer, he loved her pitch, so he gave her the assignment.