I had never seen it before, either, and I had the same reaction as you ("Huh?") to the term "Google word search definition". But I found it _is_ there, just very small and easy to miss.

1. Go to Google
2. Type in a single word (I typed "retch")
3. In the bar just above where the search results start, on the far right, I see this:
Results 1 - 10 of about 311,000 for retch [definition]. (0.34 seconds)

The word "definition" in brackets is a link to Google Dictionary's page on "retch".