Extending from this excellent question, I thought it'd be interesting to have a conglomeration of some tricky concepts in the development world explained in as few words, but as clearly, as possible.

The rules are pretty simple:

• pick a topic, any topic (virtual DOM, class extensions, etc.)
• explain the concept with brevity.

To kick this off, here are some suggestions:

• breadth-first search;
• asymptotic complexity;
• NP completeness;
• balanced weighed graphs.