In Google, if you use the search term "define:mathematics" it will provide you with definitions of the word mathematics.
How does Google determine which pages on the web are definitions for that word? (e.g. are the meta tags that you can add to a page to say that this page is a definition page?)
Best Answer
*EDIT: And looking further, it seems someone else has identified other sources so maybe my hypothesis doesn't hold. That article gives much better insight than I do. (The page is also 4 years old, but the most relevant post I could find.)
Doing a few random searches, it looks like all the answers come from the same sets of sources. My educated guess would be that Google has already determined sources which are trustworthy/reliable and have written algorithms to parse/search these resources and return these results in an order which is consistent with its other search algorithms.
I base this hypothesis on that I haven't seen any sources outside of these specific ones in approx 10 different searches:
This page offers some addditional suggestions on how to properly identify definitions in your HTML however the no conclusion was reach on how Google sources these definitions.